Proportioning Valve Question

[tcb-1]

on the front of my CPP proportioning valve, there is a interesting fitting covered by a black rubber cap.

it looks like a means of bleeding the masker cylinder without disrupting the whole system. anyone know what it is?? should I post a pic with the cap off?

[Cyclone44]

This helped me maybe it will you... http://67-72chevytrucks.com/vboard/s...rtioning+valve

[tcb-1]

hummmm,
"The one on top is the one on my '77 Sub. It has a pin in the end (under the rubber boot) that must be held in when power bleeding the brakes, from what I have read. Just pumping the pedal and cracking the bleeders *shouldn't* cause it to pop out... On the other type, the pin must be pulled out, I believe, not 100% sure on that"

so does that mean it's used when you bleed your brakes?? can someone clarify?

here's what I'm hoping it does. it allows the master cylinder to be bled by allowing fluid to enter the prop valve without introducing air.

I replaced a line from my master cylinder rear brake reservoir to the prop valve. would rather not have to bled the rear again, but I will if this fitting doesn't help the process!

[tcb-1]

anyone?

would like to figure out what this darn fitting is for. need to bleed the rear tomorrow if this fitting doesn't help.......

sorry, I'm impatient!!!

[Cyclone44]

On my stock one there is a bleeder..It's on the angled edge where that bolt is on yours above that rubber plug...If you need a picture I can take one

[tcb-1]

George,
The plug above the rubber cap is a blocked off tap for a second front brake line (if your going with two).

[RustyParts]

The valve under the rubber cover is for keeping the centering valve from shifting to one side or the other when bleeding the brakes.

Before bleeding the brakes (if your using the brake pedal) there is a clip that is placed on the end that will keep the valve from shifting so your brake light won't come on. If your using a pressurized bleeder you won't need the clip as both sides of the master cylinder are under the same pressure.

[tcb-1]

Interesting. I guess it's not what I thought it was for. Looks like a full rear bleed is in order.

Thanks Rustyparts for the insight!!!

[stope4]

First off, I'm not a brake expert, so my terminolgy and how the valve actually works may be a little off, but I'm preety sure this is what the button on the front of the proportioning valves is for.

There's a piston on the inside of the proportioning valve that can move towards the front or rear of the proportioning valve. There is a machined grove in the middle of this piston. If you remove the brake switch on top of the proportion valve, you will see that it has a spring loaded protrusion that is supposed to reside in the grove on the piston. Sometimes the piston shifts towards the front or rear and is no longer centered by the brake switch. I guess this shift occurs when the fluid pressures for the front and rear of the brake system are unequal, like when you are bleeding brakes. To recenter the piston, you can press the button under the rubber cap. Or, to prevent the piston fron shifting while bleeding brakes, you can keep the button depressed during the bleed process. Warning, pressing the button under the rubber cap requires a c-clamp or something similar. Your fingers or thumb won't be able to depress it under the extreme pressures of the hydraulics in the brake lines. I'm pretty sure that when the piston shifts, the brake light comes on.