Yall are a little off. And since none of you will take my word for it, here it is from Billavista himself via Pirate4x4... Bigblock was the closest, but still a little off.
"A combination valve is found in many OEM vehicles and may combine the functions of the metering valve, the proportioning valve, and a brake pressure warning-light switch."
Residual Pressure Valve
A residual pressure valve is a simple, one-way, spring-loaded valve installed either in the master cylinder, or inline between the MC and the callipers/wheel cylinders. They operate by keeping a pre-determined amount of pressure in the brake lines, even with the brakes released. The internal spring determines the amount of residual pressure kept in the brake lines – normally 2 PSI or 10 PSI. There are two distinct uses of residual pressure valves:
10 PSI: Drum brakes only. Because drum brakes don’t use callipers and are therefore not self-adjusting there are springs installed to retract the brake shoes away from the drum. A 10 PSI residual pressure valve is used in drum brakes to keep a little pressure in the lines to balance the return-spring force so that the shoes are maintained in close proximity to the drums. Without the residual pressure valve, the return springs would retract the shoes so far from the drums that excessive pedal travel would be required before the brakes are applied.
2 PSI: Disc brakes only. In many race cars and hot rods, the master cylinder is installed at a level below that of the callipers. As such, gravity will draw the fluid from the callipers, causing it to drain back into the MC. The result is a “spongy” feeling pedal and excessive pedal travel. A 2 PSI residual pressure valve is installed in the brake line between MC and calliper to maintain slight pressure in the line and prevent fluid drain back. Note: This valve should only be needed if the MC is lower than the callipers.
I have often read where people have advised the use of a residual pressure valve to cure some other problem - spongy pedal or excessive pedal travel (usually caused by insufficient volume from a too-small MC). In cases like this, it may feel as if the RPV has cured the problem, but it is only a band-aid, masking the real problem. Use an RPV only for drum brakes or MC lower than callipers. Do not use an RPV as a band-aid. Find and fix the real problem!
When converting from drums to discs, you will need to remove any RPV in the rear circuit.
Metering Valve
Because of the return springs present, drum brakes take more movement (pressure and volume) to initially apply than disc brakes whose self-adjusting callipers keep the pads almost or lightly touching the rotors. As such, in disc/drum brakes, a metering valve is used to prevent application of the front disc brakes below a pre-set pressure in the hydraulic system – usually about 75-150 psi – to allow the rear drum brakes to catch up. This allows the front and rear brakes in a disc/drum setup to work more evenly. A metering valve is also known as a “hold-off” valve.
When converting from drums to discs, you will need to remove any metering valve in the circuit.
Proportioning Valve
As a car brakes, weight is transferred from the rear to the front (ahh – Newton and his Law of Inertia again!). Because braking force should be applied to each wheel in proportion to the weight on it (more weight - more braking force should be applied), there is a requirement to “balance” the braking forces to front and rear wheels. Failure to do so will result in premature lock-up of lightly loaded rear brakes and resulting skid and loss of control. In fact, in hard braking the front brakes perform up to 85% of the braking! In properly balanced brakes, neither the front nor rear breaks will lock up first – braking force is “proportioned” so that they lock up together.
A proportioning valve is installed inline between the master cylinder and the rear brakes (be they disc or drum) in order to reduce the pressure increase to the rear as the brakes are applied. The pressure to the rear is not prevented from increasing – the valve just ensures that, after a certain point (the “changeover pressure”) it rises at a lower rate than it does to the front brakes. As the brakes are applied, full pressure is allowed to the rear up to a certain point. Beyond that point, the pressure to the rear is reduced preventing rear brake lock up. Most stock proportioning valves are preset by the OEM and are neither adjustable nor serviceable – they are of use only on the original vehicle for which they were designed. Aftermarket adjustable proportioning valves are available and are a must for any custom, high-performance brake system. Stock proportioning valves are of little use in modified vehicles – whether it’s the brake system or simply the weight and balance (incl. suspension height) that have been modified.
Pressure Limiting Valve
Used on some disc/drum vehicles, a pressure limiting valve performs a similar function to a proportioning valve except that instead of reducing the rate at which pressure to the rear increases, it simply limits the maximum pressure available to the rear. That means, unlike with a proportioning valve, once you achieve that maximum pressure, no matter how hard you press the brake pedal, no more rear braking is possible. Inflexible, non-adjustable, and only marginally useful on very nose-heavy vehicles, they generally suck and should be avoided.
