Originally Posted by Infinite Monkeys
...My brakes work without the valve.
Herein lies a big problem.
You have a modern M/C with two pistons on a common shaft.
IF both front and rear systems require exactly the same amount of brake fluid, your brakes will seemingly work just fine. But, what happens when one system (usually the front) wears down more than the other system? The one worn down more will require more fluid and the brakes will only advance as far as the system that stops first.
I hope you understand, as long as all your brake shoes are always adjusted to the drums your system will seem to work ok.
A combination proportioning valve COMBINES both systems by a common piston between them. The fluids never mix but mechanically, they are tied together. If one system's shoes hit the drums first, the piston moves toward the side that has lower pressure. This equalizes both system pressures while the flow is at liberty to be different. If the disparity becomes excessive, the valve turns on a switch, which indicates a brake problem.
The combination proportioning valve for drum/drum (or disk/disk) is the same valve as disk/drum except the pressure reducing portion is gutted.
If you use self-adjusters on your brakes there is no reason for a residual valve. I personally don't like residual valves because they can cause the wheel cylinder pistons to extend so much, they pop out of their bores. This was common on Tempo cars.
I suggest you invest in a combination proportioning valve because believe me, you need it. That's why ALL cars come from the factory with one.
Originally Posted by Buckaroo
Could a person use this to replace the under dash unit on a 58?
Yes, but you need a firewall bracket that extends the new booster up five inches and out (from the firewall) five inches. - Dave