DOT 5 (siicone-based) brake fluid

[leorob51]

Just over 30 years ago, a fellow mechanic who was in the National Guard, told me that Uncle Sam used DOT 5 brake fluid in vehicles that had hydraulic brakes. The only negative thing he knew about was that the rubber cups in the master and wheel cylinders swell up over time (how much time?) and have to be replaced. Otherwise there is no pitting of the cylinder bores due to moisture, like there is with DOT 3 brake fluid. Anyone out there using DOT 5 brake fluid? What track record do you have with braking performance or rubber cups swelling? As a reminder, you are not supposed to mix the two fluids. It's good preventative maintenance to flush your brake system every two to three years, if you're using the much more common DOT 3 brake fluid.

[joefrazer]

I know two club members who've used DOT 5 fluid in the KFs with less than stellar results. Both reported issues as described - swollen cups and pistons - and eventual failure of the system. They switched back to DOT3, which is what the system was designed to use.

I believe the problem with 5 is the "put it in and forget about it" billing it received. In a modern system that may be true to an extent, but in the system used by KF periodic maintenance was expected.

I would stick with DOT 3. but that is just my opinion.

[DTort96646]

I use and have used DOT 5 brake fluid in all my vehicles that do not get daily use. My 1951 Frazer and my 1954 Darrin. I have had no problems with the systems using DOT 5. DOT 5 fluid is silicone based while DOT 3 and DOT 4 are glycol based. The main difference being that glycol based fluid is extremely hygroscopic. Meaning it absorbs moisture from the atmosphere. DOT 3 and DOT 4 fluid also produces that white crystal,corrosion in you master and wheel cylinders as it evaporates and seizes the cylinders. Aside from their wet boiling points, DOT 3 and DOT 4 fluids are basically the same. For everyday use these fluids are great. The brake system gets used, any moisture is driven off, and todays brake systems are basically sealed from the atmosphere. Notice that rubber boot under the fill cap on the master cylinder? The older hydraulic systems are vented to the atmosphere, that's the little hole in the master cylinder cap. DOT 5 fluid is not as hygroscopic and does not absorb anywhere near as much moisture. Also, the silicone fluid will displace any moisture on the metal and rubber to basically protect the surfaces. The wet boiling point of DOT 5 fluid is actually higher than DOT 3 or DOT 4 fluid. DOT 5 is used mostly in motorcycles and BMW last I knew is still using it. My 1951 Frazer Manhattan sat for years unused with no loss of brake fluid or pressure, the system stayed clean and still works great. I drove my 1954 Kaiser Darrin to Gettysburg last month, 1000 miles round trip, and had no problems with the brakes. Actually had no problems at all. Now, I'm sure some folks are going to disagree, but I'm speaking from my personal experience and from over 50 as a an automotive professional. And let me just add, you do not just change from DOT 3 or DOT 4 to DOT 5. If you are going to use DOT 5, start with a clean fresh system meaning rebuild or replace all the old rubber seals, hoses, etc. and make sure the steel lines are clean or replaced. Any contamination can and probably will cause a problem.

[r1lark]

Several of my Studebakers have had DOT 5 fluid in them for over 20 years with no issues. But these vehicles had essentially new brake systems when the DOT 5 was put in - new hard lines, new hoses, new wheel cylinders, and new master cylinders were used. As DTort96646 also pointed out, for best results it's not as simple as just draining out the DOT 3 and filling with DOT 5.

[DTort96646]

I should also mention when rebuilding or repairing you brake system, use new fresh manufacture rubber brake parts, not nos 1950 whatever parts. The materials have changed and using new manufacture parts, means fresh not deteriorated parts. Same for fuel pumps, engine bearings and gaskets, water pumps, etc. just for examples.