Intermittant brakes

Any chance the brake fluid is contaminated with petroleum product such as engine oil, transmission fluid, or power steering fluid? Look at the rubber bladder seal under the reservoir cap. If it's blown up and mushy, the fluid is contaminated and the same thing has happened to the lip seals in the master cylinder.

The only proper permanent repair is to replace everything that has rubber parts that contact the fluid and to flush and dry the steel lines. That includes the new master cylinder, four rubber flex hoses, calipers, wheel cylinders, and the combination valve on the frame rail under the master cylinder. If any of those parts aren't replaced, the contamination will leach out of their o-rings or seals and recontaminate the fluid and new parts. That can happen in as little as a week or it can take more than a month to show up.

Two ways to contaminate the fluid that were common in the past were poking the reservoir cap seal back into the cap after repacking wheel bearings, and there was axle grease residue on your fingers, and wiping out a funnel used to fill the brake fluid after it was used for engine oil or transmission fluid. Sometimes we hear about people pouring power steering fluid into the brake master cylinder. That shows up rather quickly; often in a few days.

Actually, they are two different types of rubber that are compatible with different fluids. Brake fluid is a glycol product, not petroleum-based. More information can be found in automotive textbooks. I never required my students to memorize the names of the types of rubber because that wasn't going to help them fix cars efficiently. It was just important they understood that there is a difference.

I did a demonstration every year to show how critical it was to keep the brake fluid clean. I dropped two rubber wheel cylinder seals into a pair of beakers with new brake fluid and let them sit for a week. In one of them I added a drop of power steering fluid into the inch of brake fluid. By the end of the week the contaminated seal had grown by about 20 percent and was real mushy.

You can prove this to yourself too by pulling out a seal from an old wheel cylinder and dropping it in some power steering fluid for a few days. The funny thing is when you pull it out and wash it off, it will appear to shrink back to normal in a few days, but you'll find it still feels soft and sticky instead of hard and slippery.

I don't know what the ramifications are of using alcohol to flush the steel lines. To be safe, use brake parts cleaner in a spray can. It's made specifically for brake parts. It works like carburetor cleaner but it evaporates slower so it has more time to dissolve contaminants and clean it out.