Years ago when repacking front wheel bearings was a common part of a front brake job, a person's hands would be all full of grease. If you wipe your hands off real well on a clean rag, then check the fluid level in the master cylinder, you'll find those rubber seals under the cap pulled down. We pop them back into the cap with a cleaned fingertip, and presto, you've just contaminated the system with the residue on your fingers.
Often the soft metal nuts that connect a rubber flex hose to a steel brake line become rusted tight, especially if you live where I do where they throw a pound of salt on an ounce of snow. Some mechanics think they can break the nuts free so they'll spin on the line by spraying them with penetrating oil. Heat from a torch works better, but then some people apply grease to the line to prevent it from rusting again. It's a good bet some of that grease will find its way into the fluid.
Many years ago I bought an old Edsel at an auction. Within a few days it took a tank of gas to drive less than 75 miles. While driving on a granite driveway, the right front tire was skidding. Come to find out, the previous owner had drained the master cylinder and refilled it with transmission fluid! That will make the rubber seals swell and seal better, ... Just long enough to sell the car to some sucker. I had to rebuild the master cylinder, all four wheel cylinders, and replace the flex hoses.
I worked for the Auto Center at a mass merchandiser in the '80s, and we used a "bleeder ball" to pressurize the hydraulic system to make bleeding go faster. Those bleeder balls hold three to four gallons of brake fluid. We filled them from five-gallon pails with a funnel. Through our training center we heard of a guy at a store many states away who grabbed a funnel used for engine oil, wiped it clean with a rag, then used it to fill the bleeder ball. The residue from the oil found its way into at least a dozen cars. The store had to pay for hundreds of dollars worth of parts for each car, plus a real lot of labor time.
The important point is the contamination flows throughout the entire system and sinks into the rubber parts. If any rubber part is not replaced, the contamination will leach out of it and re-contaminate the system again. The only time is acceptable to not replace everything is when you see someone in the process of doing something to contaminate the fluid. If you get the master cylinder off the car right away, you may be able to save the other parts down the line. If you wait a few hours, it's too late.
Every year I did a demonstration for my students, and you can do the same thing if you get some reservoir cap seals from a salvage yard. I had two beakers filled with an inch of fresh, clean brake fluid. I dropped one wheel cylinder lip seal into each one. In one of them I added one drop of power steering fluid, mixed it up, then set them both on a shelf. One week later I washed them off, then passed them around for examination. The one with power steering fluid had grown from about 7/8" to over an inch, and it felt mushy or gooey. If that happens in a master cylinder, the seals grow past the fluid return ports and blocks them. Normally, when everything is working properly, there is a clear path for heated and / or expanding brake fluid to release and flow through the return ports and up into the reservoir. When you push the brake pedal, it takes the first inch or two to move the lip seals far enough to pass the return ports and block them. At that point all the fluid ahead of the seals is trapped and has no choice but to get pushed down to the wheels as you push further on the brake pedal. The point is those return ports become unblocked by the time the pedal still has a couple of inches to release. When the seals have grown larger from the contamination, they keep those return ports blocked all the time. That is what keeps the brakes applied, then the dragging brake gets hot, and that heat migrates through the caliper and into the fluid. Hot brake fluid expands, and since it is blocked from flowing back into the reservoir, it puts more pressure in the caliper, and that brake applies even harder.
The issue, I should explain, is brake fluid is not a petroleum-based product. It has to remain very thin and fluid down to very low temperatures. As such, it is a glycol product and all rubber parts have to be tolerant of glycol. Those parts are not compatible with petroleum-based products.
In the '80s it was also common practice to rebuild wheel cylinders and calipers as part of a brake job. Most of us would wash our hands with soap and water before installing the new rubber seals to prevent getting fingerprint grease on them. We also had to watch for leaking rear axle seals. That gear lube usually contaminated the brake shoes first, resulting in the visit to our shop, but once the seals were replaced, we still had the opportunity to get that gear lube residue on our fingers, and inside a wheel cylinder we were rebuilding.
Tuesday, October 25th, 2016 AT 8:47 PM