ALSO, does anyone have a useable strut rod for the rear of a '75 they want to sell? I do not need bushings as I have a new set of poly ones ready to install. The car has two different size rods and have over size bushings in one rod. I want to put the same size on with the bushings I have. THANKS Much.

 

        B.T.W.  the valve is on the brake system.  Left front frame rail. NOT to insult anyones intellegence.   Thanks

          Rick

 

A proportioning valve allows equal pressure front and rear up to a point, then only allows a portion of the pressure, a percentage, to go to the rear brakes to keep them from overapplying.  This is only needed with disc/drum brakes.   If it did not lessen the pressure as the rear of the car lifts during braking the rear becomes lighter and requires a lesser percentage of pressure to the rear to keep them from locking up.    Often trucks use a ride height sensor so the higher it lifts the more rear pressure it restricts.  With a heavy load on the rear it requires more braking, but does not lift as much with the load, and therfore allows more rear braking.

 

Any car with 4 wheel disc brakes does not need this.  Notice the rear caliper pistions are smaller then the front caliper pistions.  This takes care of the front/rear balance and the valve is not needed.  But you still need to know if you have a pressure loss at one end of the car, so we have the pressure switch.

 

The valve with 4 functions is a combination valve.  It contains the proportioning valve, metering valve, residual pressure valve, and pressure differential switch.   Granted it is very commonly refered to as a proportioning valve, but that's only part of it.  The functions listed in Bobs post are:

 

1. Already discussed above.

2. Holds residual pressure for two purposes.  First, to prevent the caliper pistons from creeping back in away from the caliper when driving.  Not a factor in our C3s due to the springs located behind the pistons.  With out the springs or pressure, the extra movement can cause a low brake pedal at times after cornering or driving some time without stepping on the brakes.    Second, by maintaining some pressure it ensures the caliper and wheel cylinder seals stay in contact with the surfaces and don't allow fluid out, or air in.  Not all designs need this, such as o-ring seals.

3. With drum brakes the fluid pressure must overcome the return spring pressure for the brake shoes.  This can take up to 75 psi in some cases.  Without the metering valve the front brakes would apply first, which can create poor brake performance.  They must apply evenly, or apply the rear first to keep the vehicle stable.  In many cases, 4 wheel disc do not need this function.  Without spring tension to overcome, the brakes apply evenly.  There are a few 4 wheel disc designs that still need this valve.

4. Again, already discussed above.

 

The reset can be tricky.  Once the repairs are made, the pressure diff valve may lie to you and keep the warning light on.   The switch works with a spool that remains centered with even pressure on the switch spool.  If one side drops pressure the higher pressure on the other side moves the spool and triggers the light contact, which creates a ground and turns on the light.

Some vehicles have a mild spring on each side of the spool to recenter it when pressure is relieved.  Others rely on bleeding the system to recenter the light.  Once the system is properly bleeded, one side or the other (front/rear) needs bleeded to push the spool back.  Open the bleeder, pressure drops and the spool moves.  Have someone push the brake pedal, even if a pressure bleeder was used.  Have the key on so the brake light is on.   As you are bleeding, have you helper watch the light.  When it goes out, he/she stops depressing the pedal, and lets you know to close the bleeder.    This can be done at the master cylinder.   Once everything is tight again, push the pedal a few times to be sure the light stays out.