|
|
Posted by David Sherman on Friday, June 20, 2008 at 16:29:02 :
In Reply to: Brake Question posted by Mark on Friday, June 20, 2008 at 15:54:20 :
Assuming there's enough slack in the push rod, the problem is probably a clogged relief port in the MC. I've had this on almost every one of my old rigs, and it's even come back on a truck that sat for less than a year with a little moisture in the cylinder. Someone here recently reported on a supposedly "rebuilt" MC that came with a clogged relief port. The symptoms are as you describe -- a gradual buildup of pressure until the brakes seize up. I've had it to where I had to stop along the road after every few brake applications and manually "crack" a fitting to relieve the pressure so I could keep driving.
If you take the top off the MC, you'll see a main port about 1/8" dia toward the front of the reservoir. Behind that there's what looks like another 1/8" hole, but it doesn't go all the way through. Don't assume that it's supposed to go all the way through and drill it out to 1/8". If you do that, you've wrecked your MC. There's a tiny hole in the bottom of that larger hole. A #72 torch tip cleaner just fits it, so presumably a #72 x-acto type drill bit would clean it out. If I can't push the clog out with a torch tip cleaner or very fine piano wire, I'll put muriatic acid in it and let it eat away at the rust overnight. I did that on two of them and it worked. That of course requires removing the MC from the truck and washing it very throughly after the acid treatment.
If the MC doesn't need rebuilding, best try the #72 torch cleaner or a fine hobby drill to clear it out while it's on the truck. There's always a chance that there's dirt or moisture in the MC that caused it to clog, so even if you can push the clog back into the MC, it could clog again. At the minimum, bleed the system well until all the fluid that comes out is clean and clear.
The way the system works is the MC piston has to come back forward of the relief port when you let up on the pedal and any excess pressure then escapes slowly into the reservoir through the relief port. If the port is clogged, or the piston doesn't come back far enough to uncover it, the pressure won't escape. The port is small so that when you apply the brakes, which is a fairly quick action, not much fluid will go through that port.
Follow Ups: