For years before disc were used, ALL master cylinders used a residual valve. Reason being is to keep slight hydraulic pressure against the flared lip cups, keeping the seals tight. That pressure kept the seals inflated, the springs overcame that pressure to retract the shoes.
When stopping, the shoes had to have enough pressure to overcome the springs and then move into place - about 10 lbs.
Along came discs - that same residing pressure held the pads against the discs, causing premature wear, dragging brakes etc.
Discs need to have the pressure go to 0. ANY pressure at all pushed the pads against the rotor, causing drag.
The disc's seals are square O rings, no need to keep them inflated, but the flared lips seals of drum brakes needs it.
So essentially the line pressure builds up until the drum brake's springs are overcome and the shoes begin to touch the drum. The disc's pads were already touching the rotors, no movement is needed there, so the hold-off or "proportioning" valve keeps pressure off the disc calipers. Then as the pressure is increased more, above the prop valve spring, BOTH the calipers and the cylinders get equal hydraulic pressure, hopefully stopping the vehicle properly.
The "balance" between braking forces between drums to discs is actually determined by the area of the caliper cylinders vs. the area of the wheel cylinders, and the friction materials - pad and shoe linings, as well as the friction material itself.
That's why mixing and matching brake systems from different vehicles sometimes works great, sometimes works terrible.
Master cylinders used to have the check valves in the line opening, usually a little rubber flap.
First thing ya know parts houses combined parts numbers to streamline their inventories - some MC's with the valves got installed on disc brakes - causing problems, lawsuits etc.
So - rebuilders to CYA started removing ALL check valves from ALL MC's.
Then drum brakes quit working like they should - but no lawsuits!
You've seen lots of posts about air getting in brake lines - now you know why. With no residual pressure lips seals collapse and let air in.
(Ever push on a drum brake vehicle after it sat a few weeks? First pump pedal to the floor, a pump or two later they work again. Not magic or a talented foot - the residual had bled down. Without the valve, that happens every time you step on the pedal.)
Quality parts houses still carry MC's properly fitted with the check valves - usually.
Yes, there are aftermarket in-line check valves you can buy - Jeggs etc - for custom installations when you are starting from scratch. But if you use the correct MC with the valves installed, there's no need.
Caution - If you use 2 check valves it's too much residual pressure - the shoes will drag.
Proportioning valves need far more pressure to operate than the 1-2 lbs a residual valve holds, so where it's placed is not important.
Unlike popular belief - The purpose for the proportional valve is to keep the discs from getting pressure until the shoes have a chance to move out against the drum. Disc pads don't have far to move.
If no prop valve is used, the discs start braking before the drums, making things squirrely.
The word "proportioning" valve is actually a bad word for it - a better word would be "hold-off" (Chrysler tried to use that word, everyone went nuts!) - a prop valve does not create or restrict pressure to one end or the other like the word implies. It simply "holds off" pressure to the front discs until the pressure is enough to overcome spring tension on the drum brake shoes.
Over the years an awful lot of information on the web has been based on false info.
Last edited by RRich; 08-14-2007 at 02:19 PM.