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Old 09-13-2017, 06:09 AM
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Originally Posted by Infinite Monkeys View Post
...The thing that doesn't make sense is that our cars did not come with a proportioning or metering valve... ...The block's purpose is to activate a warning light if there is a failure in the braking system...
Squarebirds were manufactured before dual-piston master cylinders were around. A single piston M/C has only one hydraulic circuit for all four brakes so proportioning and metering is impossible.

The distribution valve (I don't like to use the term, 'block') separates the two hydraulic circuits with a common piston. Yes, you get a warning switch but most folks never wire it. (This is a problem.) A valve with a floating piston between both circuits makes all the difference because both circuits RARELY use the same volume of brake fluid. The floating piston allows one side to stop flow (because the brake is tight) while the other side continues it's flow. The side that is tight simply moves the piston toward the lower pressure side until both sides have equal pressure.

In a dual-piston M/C setup, both pistons deliver the exact same volume because they are on the same spool. Without the valve, the brake circuit that applies pressure on the drums first would prevent the other brake circuit from advancing any further. Our rear wheel cylinders are smaller in diameter than the fronts, so they would lock up first. The only remedy would be to keep the rear shoes far away from the drums in a mis-adjustment fashion so the rear pistons would travel farther. This would allow the front cylinders to fill their larger-bore cylinders before the rear drums lock. No, I'm not suggesting anyone follows this crazy scheme, I am suggesting, if you have a dual-system brake setup, YOU NEED a valve (not an empty block with all the internal ports connected) with a common floating piston between both circuits.

To answer your question directly, YES, you can use the same valve on drum/drum OR disk/disk systems. The insides of a disk/disk valve do not alter or proportion pressure between front and rear circuits. I recommend using a combination valve BECAUSE:
  • A combination valve will offer metering which applies rear brakes first, then the front, then both. This is good on wet leaves or loose stones, ice, snow, etc.
  • If one circuit is ruptured, a combination valve will shut off the leaking circuit so all the fluid isn't drained.
  • The electrical switch indicates a very dangerous situation. Unfortunately, no valve can anticipate ahead of time so the alarm shows the fault already exists.

A disk/disk valve is VERY common. Recognize that it is also for a drum/drum application. I like and I use MBM products on my disk conversions. - Dave
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