Regarding the brake repair caller from 20 April 2013

After listening to Tom and Ray tear apart the guy that repaired his brakes and then had to loosen the master cylinder connection to release the pressure in order to drive home, I just have to share a similar experience. I had been making many repairs to a 1962 Ford Falcon, including rebuilding the master and slave cylinders. After probably months on my lifts (tree stumps, actually), the day came for the test drive. After about 10 miles, it seemed to be losing power and quickly came to a stop when I let off the gas. A quick inspection showed four rather hot wheels did not easily rotate. I wondered what I could have done wrong, released the brake pressure at the master cylinder connection, and drove home using only the emergency brake.
Disassembly of the master cylinder was quite amazing. The fluid return hole did not reach the compression cylinder. I thought that in my zeal to ream out the master to a geometrically perfect and mirror polished cylinder, that I had clogged the return route with filings and then managed to congeal them with excess heat to completely close hole. Perhaps I could patent my new method of welding cast iron using a cylinder reamer (connected to a very powerful 3/8 inch drill that had seen better days) - Ha! As it turned out, the fluid return route of the original cylinder had not been drilled all the way. In fact, it lacked over 1/8 inch of reaching the cylinder. It took a second drill bit after dulling the first one to get through that cast iron. The old master cylinder worked only because the pressure released past the worn out piston seal and highly pitted cylinder wall. I can’t imagine how many sets of brakes the car must have gone through when it was new.

The return port in brake master cylinders is really tiny. If it plugged up, the compressed fluid in the brake line after releasing the peddle can’t return to the fluid reservoir, and this could indeed cause the brakes to lock up. I had this happen to me one time when I rebuilt a master cylinder for my Ford truck, I got careless and a little debris got in the fluid, and the return port is where it lodged. I guess if the rubber seal leaks, that would provide a second path around, but then I’d expect the pedal to droop. I’m surprised your original master cylinder worked at all, esp when it was new.

I don’t think folks rebuild master and wheel cylinders any more do they? I think the last time I asked for brake cylinder rebuild kits for my Ford truck, the parts guy said they are no longer made. I had to buy the whole unit instead.

A problem that I see from time to time (mostly shadetree mechanics) is that they will use a clamp to compress the disc brake calipers. The thing that they forget to do is to remove the cap from the master cylinder. Compressing the calipers sends the excess fluid back into the master cylinder reservoir. The new pads will keep the calipers compressed and the fluid in the reservoir. When both sides are completed…excess brake fluid will create pressure in the master cylinder which keeps both calipers tight. The result is that the brakes will be functioning on the front wheels. This results in hot brakes until the pressure is relieved.

Interesting comment @missileman. By “cap”, do you mean the top of the fluid reservoir, that you take off when you put in fresh brake fluid to top it off? I always take the cap off b/c I’m wanting to monitor the fluid level so it doesn’t get too low, so air doesn’t get in there during the process. Or overflow. But if the cap is left on by mistake, I wouldn’t have expected that area to build up much pressure b/c there is usually an air space above it, and I wouldn’t expect the cap is air tight anyway. The cap is just there to keep the brake fluid from splashing out isn’t it? Or is the fluid reservoir cap air tight and actually part of the pressurization system? Brakes, they be complicated.