I have a 69 chevelle with all drum brakes that has me bumfuzzled. I recently started losing brake pressure, eventually to the point that the pedal goes to the floor and the brake light comes on. I've gone through several master cylinders trying to fix it, but it doesn't seem to be the problem. I'm not losing fluid. When I bleed the brakes I'm getting very little pressure at the rear wheels. Literally everything in the brake system has been replaced in the brake system, except for the proportioning(metering?) valve, (the rectangular block bolted to the frame where all the lines run through) Has anyone had one of these go bad? Any suggestions on where to find one? Please Help!

I'm pretty sure it's just a distribution block. It's basically a fancy "T" fitting to split the output from the master cylinder to the 2 front and one back lines with a pressure sensor that checks for equal pressure on both sides.

That sensor can get stuck sometimes but then your brake light would be on all the time. The only other thing that could go wrong is that one side is clogged with rust or something.

How many master cylinders is "several"? People have gotten multiple bad ones from the parts store. When you've lost pressure, have you tried pumping the pedal? Does that bring the pressure back?

Tim.

This is about the 4th master cylinder this week. I've heard of people having problems with remanufactured master cylinders, so the last one I put on is brand spankin' new. Also, the problem has been actually getting worse despite each change. After the first change the front brakes would grab and lock up with the pedal on the floor. Now nothing happens, and the brake light, which is controlled by the valve, doesn't always come on when the pedal is to the floor like it should. I know this valve is more than a distribution block and it controls the pressure between the front and back brakes. Has anyone had problems like this?

Oh, and pumping the brakes does not build up very much pressure at all.

Zac you can purchase one from Ground Up or most of the other suppliers. If you have the 2 peice type you might need to go to www.rightstuffdetailing.com. (http://www.rightstuffdetailing.com.) The right one for your car would be round,($139) if it has been retrofitted it would have a flat rectangular one. ($75)

Hello Zac,

I went through this experience on my 1970.

1. My car has disc brakes on front and metering valve located just under Master Cylinder was clogged with 30+ years of stuff. I know you do not have this valve, however your T block on the frame may be clogged. Worth a look. Once the valve was cleaned it up inside (thanks richr) it works fine and fluid gets to front brakes. Front brakes would not bleed at all with metering valve clogged.

2. Bench Bleeding the master is critical. The 2 pistons inside the master must be bled and filled with fluid before installing the master on the car. You must bench bleed it. Place master in a vise (gently) and plug both outlet ports with plugs. Pour about 1 inch of fluid in the master. Using a 1/4 inch drive screwdriver type handle (or something similar) push in the rear piston (which pushes against front piston too) and hold for 15 to 30 seconds. You will see bubbles escaping from the piston chambers. Hold until bubbling stops. Repeat process SLOWLY 4 times. Do it slowly because once you have piston chambers primed....the fluid will start shooting up as you press in the piston. Once you are done with bench bleeding, put the cap on the master and install on the car.

3. I installed the master and connected the lines, and used the bleeders on the master itself (some masters do not have bleeders...that's OK) to make sure air was completely out of the master.

4. I topped off the master with fluid and began bleeding the brakes in sequence from rear passenger side, then rear drivers, front passenger and finally front passenger side.

I installed and removed the master a few times until bench bleeding process worked. It was a real pain in the neck. Prior to proper bench bleeding, all the pedal pumping in the world still did not develop a good pedal or pressure at all. I thought I might have had a bad master too....but that was not the case.

Drop me a line if you would like to discuss.

Good luck, Joe