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Posted by Paul (in NY) on Tuesday, February 03, 2009 at 09:11:13 :
In Reply to: Why not a combination valve? posted by David on Tuesday, February 03, 2009 at 01:16:02 :
Combination Valves are typically used on front disk/rear drum brakes. The combination valve consists of 3 sections;
Pressure Differential
Proportion Valve
Metering Valve
The Master Cylinder outputs from the front and rear (separate) chambers are plumbed into the Combination valve. The two Combination Valve output ports (marked) one goes to the front brakes, one to the rear brakes.
In operation;
The Metering valve allows the rear drum brakes to engage slightly before the front disks. No pressure is allowed to the front disks until a threshold pressure is reached. The threshold pressure is low, so the drum brakes engage and then the front disks. This gives better stability in fast stops.
The Pressure differential section monitors for exactly the same pressure from both chambers of the master cylinder. If one chamber (output) from the master cylinder is weak or lost, the pressure differential valve trips and lights a warning light to let you know of the failure. After tripping, it must me reset after the problem has been resolved.
The Proportion Valve reduces line pressure to the rear brakes so in a hard stop, the rear wheels do not lock up. In a hard stop, weight is transfered to the front of the vehicle, thus the nose dive on a hard stop. Weight is removed from the rear wheels. With less weight on the rear wheels, they will lock up and skid. A Skidding wheel IS NOT braking at maximum. Maximum braking is when the tire is still rotating, just before lock up. By reducing line pressure to the rear wheels, the skidding on a hard stop is eliminated.
If you require a warning light to notify you of a Master Cylinder failure, a combination valve can be used. The Combination Valve, does NOTHING to isolate
front and rear brakes in a failure, this is done at the Master Cylinder. The Master Cylinder has dual chambers, one for front and one for rear. Your front and rear brakes are already separate with a DUAL Chamber Master Cylinder.
In a disk/disk or disk/drum system, a proportion valve works well as designed, however in a front disk/rear drum system you need 10 psi residual pressure check valve for the rear drum brakes. This valve may be built into the Master Cylinder or it may be a inline valve in the rear line. Proportion Valves, work well and are very reasonable in price. Typically in the $45-50 dollar range from Summit Racing. I use proportion valves on all my trucks. All my trucks have disk brakes front and rear.
NOTHING above applies to a Single Chamber Master Cylinder ! With a single chamber Master Cylinder, you lose the master cylinder and you loose all brakes.....period.
Paul