Brakes: Brake cyl corrosion

[jclary]

You are welcome to give me a forum spanking for not doing a topic search first...but, I'm just too lazy.

Anyway, I don't recall this being discussed before. I came in for a rest and decided to ask this question before going back out in the heat and humidity. I have never opened a brake cylinder for rebuilding that did not have some form of nasty, gooey mixture of brake fluid and corrosion between the rubber dust cover and aluminum brake cylinder pistons. If you allow it to dry, it will turn into a somewhat white flaky substance. (I have heard it referred to as "metal salts")

I use dental picks to scrape and gouge the stuff out and then a wire wheel brush to do the final cleaning. My normal way of rebuilding brake cylinders is to clean all the reusable parts a good as possible...hone the cylinder using brake fluid as a lubricant for the hone. Then I clean the cylinder with brake fluid, and make sure that all parts are coated in nothing but fresh brake fluid before assembling the cylinder.

My question ...is there any coating or treatment for the external portion of the cylinder that would discourage the formation of the corrosion that takes place between the rubber caps and the pistons? So far...I don't know of one. Seems like no matter what you do, sooner or later, corrosion forms, builds up, interferes with the operation of the cylinders and eventually, destroys the aluminum pistons.

[blackhawk]

The seals in the brake cylinder have been leaking if there is "some form of nasty, gooey mixture of brake fluid and corrosion between the rubber dust cover and aluminum brake cylinder pistons." I don't think this can be helped because eventually the seals wear out and leak, which is why we have to hone the cylinders and put in new seals. I don't believe that I have found any quite as bad as you describe. I haven't worried about the damage to the aluminum pistons all that much, as they are plentiful. I just look through my stash of used ones and find one that is in better shape, if need be. In the past, I often had the pistons in the rear brake cylinders seize in the bores even though the car was in daily use. Switching to silicone brake fluid solved that problem, since it does not absorb moisture. Maybe, for the same reason, there is less corrosion on the aluminum pistons now that I am using silicone brake fluid. I have not specifically compared them to see, but on the other hand, I don't recall this being an issue since I switched to silicone fluid. So, maybe that is your answer. ~ Dale

[PackardV8]

[QUOTE]but, I'm just too lazy.[/QUOTE]

The crud/corrosion is always caused by moisture getting into the system. Brake fluid is hydroscopic; it absorbs water from the atmosphere. The prevention, is as always, to flush the entire system at least every two years and every year in extremely humid environments.

OT, but back in '60s-70s Panama and Viet Nam, US Army clutch and brake cylinders would last less than a year between rebuilds. The only prevention was silicone brake fluid, which does not absorb water.

jack vines

[jclary]

Well...I have finished honing the cylinder. Looks good 'cept for a tiny bit of pitting just below the tiny inlet port. That is where any moisture in the system would sit and is understandable. As long as the "travel" area of the rubber cup seals is smooth, I feel good about it.

Over the years, I have rebuilt brakes in just about every common brand you can name. Still, even in cars that haven't sat for years like many of our Studebakers, you find that blasted corrosion. I have converted my '51 Land Cruiser to silicone, and that has solved many of the problems I had with it. However, this '48 I hope to have back on the road by this coming Wednesday. Don't think I'll have time to convert it just now.

I have always cleaned that blasted corrosion the best I could and went on my way. However, I thought I would pose the question here just in case someone had come up with either a cure or, at least, a way to slow it down. I suspect that even if no brake fluid leaked by the seals (and we know it does), the combination of air and close proximity of dissimilar metals like aluminum and cast iron probably create an atmosphere promoting corrosion.

I have thought about using a thin film of air tool oil on the external parts that are not exposed to brake fluid. Air tool oil is supposed to be rather inert and kind to rubber, phenolic, and supposed to be not too chemically reactive. However, we are talking about brakes (life & death) and the thought of doing something untested on a brake system is something I'm reluctant to do.

[Noxnabaker]

When a brake cylinder is new it's totally shiny inside - like chrome - you'll never get that by overhauling them unless you re-bore them to the same super-shiny surface again, otherwise the rubbers will start to wear out the minute you start using the brakes because honing does give a rough surface.
In the engine cylinders that's a good thing 'cause it holds the oil there & lessens the friction for the pistons & rings, but they're not made of rubber...

[WCP]

Wipe the piston and outer bore with silicone dielectric grease as well as the boot inside. This will help keeping the moisture out, due to the somewhat ineffective boot seals. The grease is very temperature stable and has no affect on brake fluid should a bit come in contact. Do your own tests if you are skeptical. I've been using it for many years.

[jclary]

Wipe the piston and outer bore with silicone dielectric grease as well as the boot inside. This will help keeping the moisture out, due to the somewhat ineffective boot seals. The grease is very temperature stable and has no affect on brake fluid should a bit come in contact. Do your own tests if you are skeptical. I've been using it for many years.

Thanks for the suggestion. Do you have a brand name, spec, and suggestion as to where to buy? NAPA, Advance, Auto zone? I have the cylinder honed and will probably reassemble it tomorrow. I'm going to use this rebuilt one for now, but am probably going to try to buy complete replacement cylinders soon.

[WCP]

I'm pretty sure it is Permatex brand, commonly available at most flaps - Autozone etc. Early on I used a GE brand, but haven't seen it on the shelves for many years. It has a long chemical name that I can't recall but it is a siloxane.

[Dwain G.]

Any auto parts store will have brake assembly fluid for exactly the purpose you mentioned.

[jclary]

I'm pretty sure it is Permatex brand, commonly available at most flaps - Autozone etc. Early on I used a GE brand, but haven't seen it on the shelves for many years. It has a long chemical name that I can't recall but it is a siloxane.

Thanks. I Googled the subject and found some very informative info on the subject. I have always used brake fluid, but the information I found will make me a better brake re-builder.

[christophe]

[jnormanh]

Castrol makes a good red grease fot that. Apply the grease liberally under the caputch boot and on the lip of the cylinder. I always use that technique. The brakes of my Simca 9 Aronde were rebuilt in 1988 and I have no leaks so far.

That's the trick. Moisture often gets inside the boot and starts corrosion there. As the seal cup slides farther and farther forward as the shoes/pads wear, the cup gets damaged and cruddy. A liberal application of brake grease, available at any FLAPS, and often included in a rebuild kit will reduce wear on the cup seal and will forestall future corrosion, even if the cup leaks a tiny bit. Smear a bit on the cylinder wall above the cup seal after it has been installed, and leave a glop in the boot.