Imperial Brake Fluid Information

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General information and tips from David:

First off, allow me to touch upon the generalities of brake fluids, ratings, composition, and DOT ratings.

Currently, to my knowledge, there are three commercial formulations for brake fluids; two are glycol based, and the third is silicone oil based. The DOT 3 fluids are a mixture of polyalkylene glycol ether and other glycols ( all start life as ethlyene glycol - antifreeze ); the DOT 4 fluids add borate esters to this mix.  ( Compounded correctly, these borate ester-enhanced glycol fluids can meet or exceed the DOT 5 spec. )

The DOT 3/4/5 rating is independent of the composition of the fluid, and is a rating of relative performance.  The rating refers in large part to the wet and dry boiling points of the fluids:  (all in degrees Fahrenheit )

        DOT 3   wet - minimum 284; dry - minimum 401 
        DOT 4 wet - minimum 311; dry - minimum 446 
        DOT 5 wet - minimum 356; dry - minimum 500 

Unless a great deal has changed in the last two minutes or so, silicone brake fluid is not really compatible with glycol-based( also called polyether-based ) brake fluid. The don't mix, they handle heat differently, silicone tends to be compressible under a wider range of circumstances, and they handle water differently. Plus, silicone has an affinity for the degraded bits and pieces of junk in the braking system, and tends to for a
sludge with them, eventually blocking passages and impairing brake function if allowed to accumulate for too long.

Silicone brake fluid has a number of strengths, and a number of drawbacks:

        1.) High boiling point - since it does not absorb water, there is no so-called wet boiling point 
        2.) Doesn't absorb moisture
        3.) Doesn't remove paint
        4.) The viscosity is more stable over the extremes of temperature
        5.) With the exception of some formulations used in external boots, silicone brake fluid is compatible with all current braking components

        1. ) Very hard to pour without entraining air bubbles - hence an application will generally have a softer, spongier pedal feel
        2.) Doesn't absorb water - any water in the system accumulates in the lowest point of the system and stays there, concentrating rust
        3.) Whereas glycol fluids begin to compress near their boiling points, silicone fluids begin to compress at around 300 to 350 degrees Fahrenheit.
        4.) Additives in the fluid can vaporize at comparatively moderate temperature, increasing the spongy feel.
        5.) Silicone fluids expand significantly when hot.
        6.) Functionally incompatible with systems which have held glycol-based fluids for any length of time, requiring flushing and seal replacement     (Note that there are counter opinions on this that state that the modern silicone formulations are in fact compatible with only a flushing, rather than a complete reseal. )  The actual DOT specification requires chemical compatibility, so as far as that goes, the two fluids won't cause reactions if used in the same system, but they certainly won't mix, either.
        7.) Functionally incompatible with anti-lock brakes.  In the first place, the silicone fluids tend to be more viscous, which can cause problems with the timing of the pulses, which are intended to work with the thinner glycol-base fluid. This sometimes leads to damage to the ABS valving.
Secondly, the rapid pulsing necessary to anti-lock function tends to cause cavitation in the fluid, as the tiny bubbles collapse and coalesce into larger ones, and then collapse and reform into smaller ones.  This tends to nullify the ABS effect, can diminish the actual effective braking to a dangerously low level, heats the fluid leading to further sponginess, and can damage the ABS controller.  Thirdly, silicone brake fluid tends to foam when expressed from a small orifice under pressure.  This, of course, reduces its hydraulic effectiveness to nothing in the area affected.
        8.) The silicone tends to attract and bind with the fragmentary wear products of the rubber components in the brake system, creating a gelatinous sludge, which can block fine passages, particularly in ABS systems.

I have read of the experiences of a number of members of my Triumph list who have gone to silicone brake fluid - other than flushing and resealing the system, and the seemingly  permanently slightly spongy pedal, most reported that they were satisfied.  One very real issue with these cars is that the seals are not what the could be in the brake and clutch master cylinders, and _every_ owner has stories of leaking MC seals destroying the paint on the firewall, so this aspect represents a very real value.  Also, these are usually weekend cars, for pleasure driving only, so they don't see a lot of
brake system heat.

Having torn silicone fluid down, who uses silicone fluid then?  The sole major user, to my knowledge, is US Government, mainly the armed forces,
although the Post Office is a big user, too.  They use it because it is stable and predictable over a wider range of temperatures, has a high boiling point, and doesn't absorb moisture. ( It does not appear whether the government cares about the paint issue. ) I can find no evidence of any other major benefits that the government expects from the silicone fluid.

In a word - do not use silicon fluid in systems with ABS - water droplets can damage the ABS pump if caught in the pump when ABS operates. 

I use Castrol LMA or MB fluid (Pentosin brand) - both are DOT 4 rated . . .

And always use pressure bleeding with ABS brakes - you simply cannot get a good brake fluid flush by pumping or vacuum method.

Question from Tom (1956):

I will have to bleed the brakes on my '56 very soon and I was wondering what the difference is between "modern" brake fluid and the older silicone type. When I did the first brake job, I sent away for the silicone brake fluid from a specialty shop (Andy Bernbaum I think) and it cost about $16.00 per quart, plus shipping. Is that the right way to do it, or can I use the DOT 3 fluid available from the local supply store?


From Tony:

[I can't pretend to be anybody's authority, but I switched my '61 convertible to silicone brake fluid specifically because of its non-corrosiveness (NEVER let older-style brake fluid get on your paint!) and its indifference to water (older-style fluid absorbs water, encouraging rust and reducing brake-effectiveness) - My '61 coupe's brakes always needed something or another to keep them from getting spongy - I've had zero problems with my convertible. It strikes me as a no-brainer - swap it out properly and you'll never look back! 

From George:

Yes I did too. More stability. A little more pressure needed when applying the brake. But better for the car. No water, therefore, less corrosion.

Go ahead and do it.

Never mix the silicone with anything else!!

I did mine after a total replacement of the braking system.

From Brett:

I agree with Tony. Regular DOT3 fluid can absorb tremendous amounts of moisture. This moisture will rust out the wheel cylinders on any car that sits for a period of time. This is a real problem here in Florida because of the humidity. DOT5, or silicone fluid, was developed by the military for vehicles that go through periods of disuse. It's also non-corrosive as a bonus.

If you drive your car frequently (several miles, a few times per week), then the heat build-up from regular use will dissipate moisture in your brake lines. There is also DOT4 fluid that is supposed to absorb less moisture than the DOT3 does.

Some people don't like the "feel" of DOT5. They say it's a bit spongy. DOT5 can also cause connection problems with the hydraulic brake-light switches that are on 50's and early 60's Imperials.

[Incidentally, I believe that 1961 Imperials were the first to use the non-hydraulic brake-light switches - they were a mid-year change, unless my parts guy was wrong. - Tony]

But, these problems are rather minor when compared to the cost of a full set of shoes and wheel cylinders.

Follow-up from George:

>DOT5 can also cause connection problems with the hydraulic brake-light switches that are on 50's and early 60's >Imperials.

So that's why I don't have strong brake lights! Have replaced the sender before and now must do it again.

From Tony:

RE: Brake light switch. If you are using DOT 5 silicone fluid you should consider replacing the pressure switch with a mechanical switch. Several reports have stated that DOT 5 will cause the pressure-type switch to freeze up, therefore the mechanical switch should be considered. Wes [Wow - I hadn't heard that, and it would explain why I have had problems. The wiring-harness is different, unfortunately. I'm going to have to install (and hide) an "extension cord" to take the wiring inside, near the brake-pedal, rather than out in the engine-compartment. 


I've experienced the problem with several Studebakers, the solution the Studebaker Drivers Club came up with was to pre-load just the switch with lower # dot 3 or 4. No more problems. I did this several years ago and know several others that also did it also. The stuff won't mix - it stays in the switch.

From Paul:

I have had the brake switch problem with silicone fluid and changed the brake switch to mechanical on a '56 New Yorker. I still believe that the advantages of the silicone fluid make it a worth while change even if you have to engineer a mechanical brake switch.

Tips from George and Ron:


Silicone brake fluid and regular brake fluid should NEVER be mixed! The are incompatible, and doing this can cause BIG problems!

If you are going to switch over to DOT-5 silicone fluid, you must completely remove all of the regular stuff- it would be best to disassemble everything, and clean the parts with denatured alcohol or spray brake cleaner.

Personally, If my vehicle's brakes were doing fine, I would simply use a suction bulb like the old ones for batteries, or a turkey baster you stole from the kitchen to suck out the old fluid from the master cylinder. Then refill with fresh fluid and bleed the brakes until the fluid comes out clean.

Do this once a year, and one shouldn't have much worry over corrosion, etc. 


Most likely at the time of conversion to Silicon the rubber parts are in bad shape and SHOULD be replaced anyway, but if they are not worn/leaky/tired or about to fail anyway, a Silicone conversion WON'T HURT no matter how it's done.

It would ALWAYS be best to do a rebuild, but having used Silicone for 15+  years, we are a Gov't contractor (post office) and that's all they allow to  be used, I can tell you that no matter how poorly I have flushed a GOOD old  dot 3/4 system before conversion they ALWAYS HAVE lasted much longer afterwards.

Follow-up from Dick:

I have converted about 15 cars to silicone fluid over the years, with generally good results. I disagree with the posting that said it is OK to just flush and replace the DOT-3 with DOT-5 though. The DOT -3 and DOT-4 fluids are alcohol based, and as such tend to expand rubber products. If you simply replace the fluid with DOT-5 (silicone) fluid, you are very likely to get some leaks from the shrinkage of the rubber parts back toward their unexpanded state. Possibly if the parts are nearly brand new you can get away with this, but I have found the only foolproof way to do it is to replace all seals and hoses, plus the brake light switch if hydraulically operated. I blow the lines dry with air and make sure all the old stuff is out of the system, then rebuild with all new rubber parts. Some cars I did this to 30 years ago are still untouched and fine.

Question from Chris:

I was wondering if any of you out there has used silicone based brake fluid in your brake overhaul projects. Any wisdom, comments, etc. is greatly appreciated.


From Ron:

I have 25+ years experience using silicone brake fluid, over 10 years as a USPS contract repair shop serving 2 PO's with over 300 vehicles. I am a shop owner, and I use this in ALL my NON ABS cars. I have used it in just about all (wrongly advised?) ways possible; in totally renewed systems in partially renewed systems in old drained, flushed, inspected systems Mixed in over old dot 3
etc, etc, etc.

In my own work car, a Honda Accord, when I did the front brakes w/new calipers/pads only, all I did was suck out the master and put silicone in over the top (the rears are still on dot 3 but they share a common reservoir so there is some mix) Did that 3 years/80K ago NO PROBLEM

I use(d) it in ALL my Chrysler Corp cars including my last '67 Imp CC

Don't be worried about reports of a leaking tendency, those systems that leak have other problems/conditions that cause the leak.
If it doesn't leak on Dot 3 it won't leak on Silicone.

Personally, ALL ( 20+) of my non ABS cars/motorcycles have Silicone, it's near the first thing I do to a new arrival, and on ALL old cars when I do a restore brake job.

From Paul:

For me it has been the only way to go. Some of my cars sit for long periods of time. In this area, moisture in the system is common and highly destructive. I now run all of my cars, even daily drivers on DOT 5. The 10 + years that I wasn't able to run my Imperials chiefly was due to brake problems. Now that is no longer an issue.

Some people have complained of spongey pedal with DOT 5. This has not been my experience. In fact all of my cars have better brakes now, than they ever did with other fluid. That could be due to my replacing the entire system on all cars. I have heard that this is no longer necessary to make the conversion, and that all you need to do is flush the system completely with DOT 5 until all fluid bleeds purple. I have not done this, so I can't say. They use to recommend all new parts, so that is what I have done.

From Steve:

I use silicone brake fluid in all of my collector cars for several reasons. It isn't corrosive to paint, doesn't absorb water and is alot easier to work with than conventional brake fluid. I've never had any problems with it.

Question from Gary:

Any adverse reaction to using dot5 in place of dot 3? I recently replaced the entire brake system including master cylinder, brake lines, calipers and rebuilt rear brake assembly. The system was "sterile" before adding dot5.  After about two months (maybe sooner) I developed a leak in both the front brake calipers (BUDD). In the seam that joins both sides of the caliper together on both calipers. Hence, the fluid leaked and completely ruined the pads as they are soaked through. Could this be caused by the use of dot5? In that time the car has only been driven about 75 miles.  Napa is replacing the front calipers and the pads. I don't want to introduce them to dot5 if that is the reason the seals failed in the calipers. If the  replies come back negative in dot5 I will first have to flush out the existing dot5 and replace with dot3 or 4.


From Jack:

I rebuilt the complete brake system on my 61 five years ago, using silicone brake fluid. I noticed a slow, constant loss of brake fluid (filled up master twice a year) with no signs of a leak.  I lost my stop light switch (mounted on the master cylinder) I replaced  it with a brake pedal mounted stoplight switch rerouting the two stoplight wires. The ability of silicone brake fluid to retain air was taken care of by slowly heating the silicone brake fluid on my stove to release the air before putting it into the system. (make sure your wife or girlfriend is not around) My brake pedal is high and firm.  Now, five years later I wouldn't do it any other way. Putting  brake fluid  in my 61 is a pain the a** because of the location of the master and booster. Not worrying about spilling brake fluid on my paint is one problem  I don't  need. Silicone brake fluid will not harm paint.

From John:

I have converted a couple of my cars (a '60 and a '63) over to D0T5 when I rebuilt the brake systems. Based on my experience, I agree that DOT5 is the way to go and would never go back to the glycol fluids. I've noticed that DOT5 is prone to formation of tiny air bubbles if you agitate it or pour it fast from the bottle. This is why it is difficult to get hard pedal (especially in the pre-'63 cars with the dual front wheel cylinders that are cocked off vertical -- can be a b*tch to bleed no matter what fluid you are using). After a while the tiny bubbles combine to form larger ones and you get more pedal travel. I hadn't thought of trying to de-gas the fluid by heating it up so will have to try this on the '60 since it's due for a re-bleed (the pedal is about 3/4 the way to the floor on a hard stop). I've, also found with DOT 5 that pressure bleeding works better than manual bleeding (if you manual bleed it has to been done s-l-o-w-l-y) but have had lousy results with vacuum bleeding. I have had no problems with leaks and the hydraulic brake light switch is still working fine after 2 years. The '63 was a slam dunk needing no special bleeding technique and I had firm, high pedal on the 1st try with manual bleeding. About 5-6 years now and the master cylinder looks brand new. 

From Bill:

I have always been told that to use the Silicone brake fluid, the system must be entirely new including the master cylinder. Seems like you did that so I would think you may have had a pair of faulty calipers. Myself, I would stick with DOT 3 and not take the chance. Not sure what you should do to switch back short of replacing all components again.

From Bill:

DOT 5 will is unforgiving, it will find every way to leak out of a system. To the extent that some car manufacturers do not recommend it any more. This stuff is used also in hydraulic clutch mechanisms. It also has been know to distroy brake light switches and to bol at lower temperature levels than other fluids. Today I have 5 cars with it and am wondering if I shouldn't go back and change it. Some of my cars have had it for 10 years with only two problems. 1. I had to reroute a brake line because it was to close to the exhaust and I would get a soft pedal after long driving. Moving it solved the problem, I guess it was boiling in the line. 2. I have had brake switches fail in my older cars which I understand is a common problem with DOT 5. Now I preload the switches with DOT 3 or 4. No more problem there. 

From Brett:

When converting to Silicon DOT 5 brake fluid, ALL RUBBER parts need to >be replaced as the DOT 5 & DOT 3 fluids react to each other. I have heard this issue mentioned before, but "How to Restore Your Collector Car" by Tom Brownell (1984, ISBN 0-87938-174-4) tells a different story... The book explains that DOT5 was developed by Dow Corning as a solution to polyglycol (DOT3). After all, the military was an immediate customer with many vehicles that sit for long periods of time... even the DOT3 in trucks parked in Arizona absorbed 8.4% water over two years. Direct from the book: "... it is possible to gain most of the water-free advantage of silicone brake fluid by flush-filling the brake system with silicone. Since silicone fluid is compatible, though not miscible (i.e. will not mix), with standard polyglycol brake fluid, the glycol residue that remains in the system will not affect the performance of silicone fluid." "...siphon or scoop all the old brake fluid from the master cylinder and wipe the chamber or chambers with a clean, dry cloth. Next, refill the master cylinder with DOT5 silicone brake fluid. Allow several minutes for the fluid to settle so that air bubbles can escape, then purge the system of its remaining polyglycol fluid and bleed any air that is trapped in the lines." Bleed the system according to your shop manual. You'll see a change of color when the fresh DOT5 comes through. "Although flush-filling a car's brake system with silicone fluid reduces corrosion, it is impossible to remove all the glycol fluid by this method. A better approach, especially if the brake system need maintenance anyway, is to dismantle and rebuild the wheel and master cylinders, replace the brake shoes and corroded lines, then refill the reconditioned system with fresh silicone fluid." I doubt the publishers of this popular book or their attorneys would allow the instructions above if there was any chance DOT3 & DOT5 in the same system may cause a brake failure. In addition, I have converted at least four of my own cars (in the 1957-61 range) to DOT5 using the method listed here without any problems. In fact I haven't had to replace any wheel cylinders once silicone was in the system... six years. The only issue DOT5 has caused me is the premature failure of hydraulic stop light switches.

Tips from Kerry and Peter:

 The consensus seems to be that if you are redoing your brakes, you should flush and replace your old DOT3 fluid with DOT5 (silicone). Advantages are mainly that silicone will not absorb moisture that leads to corrosion of lines and seals.

However, in the last couple of months, letters to the editor of Skinned Knuckles (a great magazine for people who get their hands dirty) indicated the dark side of DOT5.

Apparently leaky power brake boosters can cause the brake fluid to be drawn into the intake manifold through the vacuum lines and from there into the combustion chambers.  When heated there are at least two bad things that happen.

(1) Silicone turns into an abrasive dust when burned.  Abrasive materials are NOT conducive to engines.
(2) The dust tends to accumulate and foul spark plugs.  This shows up as white powder on the electrode.

The moral of this story is that unless your power brake booster is in perfect condition, Silicone brake fluid may not be the best alternative.  After all,
your old Imperial has lasted quite a few years with traditional DOT3 based fluid.

One final thing is that any silicone products is very bad for any painting you plan to do.  Even a small drop in the air can cause 'fisheyes' in paint months later and at the other end of the shop.  Personally, I try to avoid silicone in any liquid form (including WD40) in my car shop.  RTV compounds are something else and do not cause a problem.  However, most people will tell you to be careful using RTV type gasket sealers and not let excess get into the internal engine areas.  The result can be exactly the same as with DOT5.

As for me and my house, I will stick with DOT3.

Follow-up from Peter:

I have some personal experience with DOT 5 brake fluid and did some research after experiencing problems.  I also had a magazine article detailing the facts about brake fluid but I haven't been able to find it this evening.  I'll keep looking, but here are the basics:

Most people know that DOT 3 and DOT 4 brake fluids are glycol based while DOT 5 is silicone based.  Many people, however, don't realize that any brake fluid has an additive package just like gasoline and oil.  The two additives that come to mind are "swell" agents and anti-foaming agents.

Swell agents react with the rubber components of the brake system to keep them supple and promote good sealing.  Since DOT 3/4 and DOT 5 have different bases, different swell additives must be used.  The problem is that old brake systems use rubber parts that are NOT compatible with the DOT 5 swell agents.  I can't remember the specifics, but DOT 5 fluid additives used with older rubber parts cause the rubber parts to either not swell and leak or become gummy and leak. Either is bad.  Newer production rubber parts are made from modern materials (EPDM???) that are compatible with DOT 5 swell additives so this isn't usually a problem -- unless you are attempting to reuse old rubber parts or very old NOS rubber parts.

The article also pointed out that plain old air is a big problem with DOT 5 fluids.  While DOT 3/4 fluids hold a maximum of say 5% dissolved air by volume, DOT 5 holds 2 or 3 times that much.  This creates the problem that I experienced -- no matter how many times you bleed the brakes you never get a good, firm pedal.  I tried and tried and tried with my 70 LeBaron but the pedal always felt
spongy.  When I bled the brakes, I couldn't find any air.  It was driving me nuts.  Draining, flushing, and refilling the system with DOT 3 fluid solved the spongy pedal problem.

Another problem I ran into was someone accidentally adding DOT 3 fluid to a system filled with DOT 5 fluid.  This was a nightmare.  Some nasty chemical reaction occurred causing the fluid to turn into something resembling liquid rust.  It took all new wheel cylinders, a rebuilt master cylinder, and a lot of Brakleen to clean up the mess and get things working again.

DOT 5 fluid did NOT turn out to be the silver bullet I had hoped it to be.  IMHO, you'd be much better off following the maintenance schedule recommended by European car manufacturers -- flush the brake system and refill with new fluid every 2 years.  This gets the moisture out of the system, greatly reducing corrosion problems.

I didn't know that WD40 contained silicone.  Does anybody know if CR 5-56 does? I've heard for years that WD40 is crap and 5-56 is good.  I never understood shy, and, unfortunately, very few places seem to stock 5-56.

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