Disc Brake Proportioning Valve

[rockphil]

My '69 Z28 has the factory front disc and rear drum brakes. The front disc brakes used a proportioning valve which has a boot/button on the back side of the valve. It's my understanding that this a "pressure button" which allows you to basically bypass the function of the proportioning valve to allow for bleeding of the front calipers.

The button on the valve that was on the car when I bought it didn't seem to work - when I pressed on the boot I didn't not feel any button release or movement. I was not able to bleed the front calipers.

So - I bought a new "original type" proportioning valve with the boot/button. Before installing it I tested the button to see if I could feel it release - it was hard as a rock with no movement. I presumed at this point that the valve needed some pressure in the system for the button to operate. So I installed it. Guess what - I still am not able to feel in movement in the boot/button when I press it. I am still not able to bleed the front calipers.

Does anyone know how the boot/button on this original type proportioning valve is to supposed to operate? What am I supposed to do that will give me the ability to bleed the front calipers?

Your wisdom is appreciated.

Rocky

[BlackoutSteve]

In the 69 service manual, Page 5-29, it says the pressure regulating/metering valve can be depressed by hand or by clamping.
Mine was very hard to press by hand so I clamped mine too.
I also pressure bleed by brakes using a modified pressure weed sprayer at about 10psi. So far, without driving the car, the pedal is good, the car holds well, and all air was easily bleed out at each wheel.

[JohnZ]

That's not a proportioning valve - it's the "metering" or "hold-off" valve; its function is to block any fluid flow to the front calipers until the valve sees 30-40 psi, which opens the valve. Bleeding by pumping the pedal should open it with no problem, but if you use a pressure bleeder that applies less than 30-40 psi, that isn't enough to open the valve and you have to manually depress the button at the rear of the valve, as noted in the Chassis Service Manual.

[rockphil]

Thanks John,

Unfortunately - the button will not depress on my old one or on my new one.

I am using a vacuum system attached to a compressor (similar to a mytivac). It pulls some fluid through but not much. That's why I was trying to use the release button which is supposedly what would allow the line to open up for bleeding.

Any thoughts on why the button is not operating correctly?  Do I need pressure in the rear lines for the button to work?

Also - I have heard the brake components called by different names.

Like you - I thought the valve attached to the power booster was called the meter or hold off valve. The company I bought it from calls it the proportioning valve.

I always thought the the proportioning valve was the one located directy under the driver's seat. But they call this the "hold off" valve. 

[BlackoutSteve]

Also - I have heard the brake components called by different names.

The GM Service Manual calls this same part both a pressure regulating valve and a metering valve.

[JohnZ]

Like you - I thought the valve attached to the power booster was called the meter or hold off valve. The company I bought it from calls it the proportioning valve.

I always thought the the proportioning valve was the one located directy under the driver's seat. But they call this the "hold off" valve. 

NONE of the vendors call it by its correct name - they ALL call it a "proportioning valve" and it's not; as you noted, that's the one on the side of the subframe below the driver's door.