This is a copy and paste from another post, I am posting it cause I see another guy is not using a prop valve, which I personally am agasint (but to each his own).
First off is the duel job of a proportioning valve. I explained the how it holds pressure but it has a much more important job that I like to stress. You guys remember the old vehicles that came with single piston master cylinders. As we call them, suicidal master cylinders. Problem with these is once the 1 cup failed all your brakes failed, time to downshift and hit the e-brake and pray.
So along comes the duel master cylinder to solve this problem. If 1 cup fails then half your brake system failed and the other half will stop you. All good in theory but it’s only half the story. Those 2 pistons in the master operate together. If 1 fails they BOTH fail to the point where you can’t stop the vehicle. You guys have all heard this story right?
“I was driving down the road and was coming to a stop when my brake pedal hit the floor. I panicked and pumped my pedal and was barely able to come to a stop. Scared the crap out of me”.
This is what happened.
1. A master cylinder cup failed and both pistons followed each other and you lost all brakes.
2. Both master cylinder lines are attached to the proportioning valve; these lines put pressure on both sides of a piston inside the valve that’s sole job is to do nothing when you have pressure on both sides.
3. One side of the master cylinder had pressure (the good side) the other side did not (the failed cup). So the piston’s second job came in. That’s to be slid over by the pressurized side to block of the port to the bad side.
4. Now your good side of the master cylinder will operate enough to stop the vehicle. The bad side is blocked off and the good side can put the pressure out to your brake system instead of through the bad piston.
I stress this for a reason. I see many guys removing their proportioning valves and replacing them with metering valves. When you do this you just went from a duel master cylinder to a single (suicide) master cylinder. If you feel you have too much pressure and would like to put on a metering valve to fix it, go ahead. Just please leave the proportioning valve on, don’t remove it.
Note: If you need a metering valve there’s a reason. They where designed for racing vehicles not as a “fix” to your brake systems problems. Best bet if your modifying your brake system is to do what GM, or Ford, or the like did. Decide on a front caliper, see what the manufacturer used for the rears and use the same sizes. They did all the testing and engineering for you, take advantage of it.
