Ghost in the Machine? Brake Problems

Went through the first 10 pages and didn't see this question asked. I'm only going on what I've been told about this 87 B2200 2wd auto truck. 1st it is a US MAIL vehicle. Several weeks ago she replaced the brake master because the pedal was going to the floor. The truck started applying the brakes on its own or they stayed applied one or the other. Since the work started it has had one front caliper, front pads, rear shoes and booster replaced as well as a second master cylinder. Any suggestions as to where to look for the problem? Trying to help a friend.

Make sure that whoever did the brake work bled the brakes correctly. There is a bleed valve on each wheel and you need to bleed all 4 of them to be sure that the brake lines are free of air.Second, the rear wheels are drum brakes and they have to be manually adjusted.I also think that the rear drums have an adjustment that will tighten the brakes when you back up and step on them hard. If that mechanism isnt installed correctly it could cause the problems you are describing. Hope this helps. BJ

Make sure the pushrod into the master cylinder is adjusted properly, it sounds like you need to shorten it, that it's always applying some pressure to the brake system. When I first swapped the master cylinder on my B2200, the geometry was quite different, and after a few miles, the brakes would lock up. So I had to adjust the pushrod shorter. You don't have to re-bleed for this, just loosen the two nuts holding the master cylinder on, then pull it gently forward so you can get two wrenches on the pushrod, loosen the locking nut, then turn to adjust, the retighten. You may need a needle-nosed vise-grip, preferabbly the bent-nose type.

One other suggestion. An 87 model truck probably has the original brake lines on it. They can become brittle with age and leak. I would suggest getting a set of stainless brake lines from someone like Goodrich and installing them. They wont wear out or deteriorate, and have a smaller inside diameter which leads to better pressure on the brakes. BJ

Cusser: I have yet to see this truck. But when I first heard the problem that was my thought exactly. I didn't know if there was an adjustment or if the rear brakes are self adjusting. I've never really been inclined to work on Mazda B's. That speaks rather highly of them since I've had two in my long line of vehicles. Neither offerd any relyability problems while I had them. Since this problem arose after the master was swapped; I've got to be inclined to think along those lines first. Or there may have been some bastard mid year swap from one part to another. And I have seen bad numbers asigned to aftermarket stuff. Those are the ones that make you pull your hair out. But either way something like that could affect a change in the rod length.

The guy she's got working on it has been at the same place about 20 years so he can't be totally stupid. I assume:uhoh: he has tested it and experienced the problem. I would think a braking problem severe enough to lock the vehicle down would envolve enough heat and smell to isolate the problem to the brakes or even particular ones. I had a Chevy Astro that had a tranny let go one night and slam locked up the back wheels. Just like if the rear brakes slammed on. When it slowed down and changed back to 1st I could go as long as she didn't upshift. By the time I got a mile from my house I only had reverse. It was late at night in the country so I backed it up to the drive at my house. War story but an example of how something could be misdiagnosed.

Anyway thanks for the responses.

OK...Still second hand info; but my wife told me today the problem occurred due to a misadjusted, mastercylinder, push rod. She said it was adjusted too short??? I don't know if something was lost in the telling but I'd of thought too ong would have caused the trouble. Anyway the mechanic R&R'd the brk master, 2 cals, frt pads, Rr shoes, booster and the brk lines. She finally took it somewhere else and the new mechanic found the problem. You can bet there's one PO'ed Postal Carrier in my county. It made me feel good to have told her this four weeks ago. So there is a good "I told you so," coming.

Great that it's fixed. I think wife mis-interpreted the mechanic, that in reality the pushrod was "too long" and needed to be adjusted "shorter". If the pushrod was too short, free play would increase and braking might be significantly less.

Great that it's fixed. I think wife mis-interpreted the mechanic, that in reality the pushrod was "too long" and needed to be adjusted "shorter". If the pushrod was too short, free play would increase and braking might be significantly less.

Yeah. Once again we're on the same page. I figure it was too long and had to be shortened. Ya know how things get misinturpreted by the time they get told a third time; especially by people who are ignorant of a mechanism?