Mid Year brake system problem.

[Jerry Clark]

Hi Everyone:

We have a brake problem . Car = 65 365 AC convert from the collection. Converted to silicone DOT 5, compatible with DOT 3 & 4, flushed the system, rebuilt two leaking calipers, . The brakes are LOCKED. Can't relieve the pressure. Rebuilt the brake master, (no power booster)replaced all four caliper hoses, still locked so badly the pads can not be removed. If a bleeder is cracked the pressure is released. Secure the system, apply the brakes and they lock again. This is a new one on me, I'm out of ideas, HELP.

Jerry

[Terry M.]

Re: Mid Year brake system problem.

Jerry,

If all four calipers are locked - problem has to be with master cylinder. Especially in light of apparent lock up on application of brakes. Master cylinder is not releasing pressure for some reason.

I note your post indicates DOT 5 compatible with DOT 3 & 4 - I don't think so. I don't think there is any compatibility there, but I would not expect that to be the root of the kind of problem you are experiencing.

Disassemble master cylinder and compare order of assembly to diagrams in Chevrolet Chassis Overhaul manual. Check seals for swelling - DOT 5 should not cause swelling, but if some petroleum product was used to flush system there could have been contamination due to that product. Not sure what product to use to clean out silicone, but alcohol is appropriate flushing product for DOT 3 & 4 fluids.

Terry

[Rob Brainard]

Re: Mid Year brake system problem.

First of all, if you will read the info. on the DOT 5 bottle, I think you will find that DOT 3 and DOT 5 ARE compatable. Maybe not a great idea to mix them, but shouldn't be a problem. Locking up -- check to make sure the brake pedal is free to return and/or that the adjustment [if there is one] is correct. One way to check to make sure that the adjustment is correct is to remove the MC cover and depress the pedal slowly. If you do not see a squirt of fluid the master is not returning to it's normal [unapplied] position.

[Gary Schisler]

Re: Mid Year brake system problem.

I agree with Terry that the problem appears to be in the master cylinder. If the problem occurs in more than one brake, what else could it be. An inexpensive way to check this out, after the MC rebuild, would be to go back to Dot 3 or 4 fluid and see if the problem returns. I don't use DOT 5 (silicone) fluid in my 66 with stainless brakes. I just don't care to check out the compatibility of DOT 5 and today's rubber compounds. Either way you go, I would spend the money and flush the current fluid with whatever you choose to use, DOT 3 or 5, then refill with your choice.

Gary

[Roberto L.]

Re: Mid Year brake system problem.

Could be brake hoses clogged, it happened in my car and have to change old brake hoses. Symptoms were the same. Roberto, NCRS #30019, RMC

[G B.]

Master cylinder problems

This has been my experience.

I've come across several brake systems in which the owner has mixed silicone with regular brake fluid. Usually this was done accidentally when the lines weren't completely flushed before installing the silicone. Two owners replaced their master cylinders and used silicone to top off their DOT 3 systems. In none of these examples did the mixture of the two different fluids form any gel or attack the rubber components.

As early as 1969 some '65 master cylinder rebuild kits came with a discharge check valve. It must go on the end of the piston in some other application of the kit, because it sure won't work in a '65 Corvette. It will cause the brakes to lock once applied. I know.

If the master cylinder piston doesn't retract fully it will also cause residual system pressure to keep the brakes applied. Sometimes the piston can hang up in the bore, especially when the master cylinder has been sleeved incorrectly. You may want to check the amount of free play in the push rod. It should move forward a sixteenth or so before contacting the back of the master cylinder piston.

[Duke W.]

Re: Mid Year brake system problem.

DOT 5 silicone brake fluid may be "compatible" with glycol based DOT 3 and 4 fluid, but it is not "miscible". If you try to mix silicone with glycol you are asking for trouble. You cannot successfully change to silicone using the "bleed to flush" method, either manually or with a pressure bleeder. Globs of the old dirty and moisture contaminated glycol will remain. Silicone and glycol have significantly different physical properties (specific gravity, and viscocity), and they will not mix. The only way to convert to silicone is to completely overhaul the system including thoroughly flushing and drying the hydraulic lines with ALCOHOL. Using a petroleum solvent will leave a residue which will rapidly swell and disintegrate the rubber seals and cups.

Glycol based fluids must be changed every two to three years to slow down internal corrosion. Glycol attracts water like a magnet attracts iron, and there is no way to keep water from eventually contaminating the system. Silicone will not absorb water, so it can stay in the system longer, but should still be flushed every five to ten years.

Glycol based fluid is an unfortunate legacy of the first automotive hydraulic brake systems in the early twenties. Back then all manufactured rubber products were incompatible with petroleum, and glycol was the only available choice. Power operated equipment of the era, such as construction equipment, used cables and pulleys. Modern hydraulic systems were developed in the forties to replace cables as aircraft got bigger. Since we were cut off from the natural rubber supplies of SE Asia the chemical industry developed synthetic rubbers from petroleum and simultaneously developed synthetic rubbers which were highly resistant to petroleum attack. Thus all modern hydraulic systems use petroleum based fluids, but we are left with the glycol legacy in our cars. The industry and DOT have never wanted to bite the bullet and change fearing that older cars would be topped off or filled with the "wrong" (petroleum based) fluid which will rapidly destroy the seals and lead to brake system failure.

If you are completely overhauling your system including the hydraulic lines and will start with a completely clean and dry system, consider silicone fluid. Otherwise, stick with glycol and change it every two to three years.

There is no substitute for a thoughful and DOCUMENTED maintenance program. That's why airplanes last virtually forever. Even cars in storage must have maintenance beginning with proper storage preparation which should include a brake fluid change before they are placed in storage.

Duke