This is the valve mounted below the master cylinder on most '69-'70 GM A-body cars with disc brakes. Just curious what it does. It is not on '71 and up GM A-bodies. Did GM deem it superfluous?

 

No, GM designed the distribution block to incorporate it as of 1971, so that there is no longer a separate hold-off valve.

The valve serves to equalize front and rear braking by delaying the application of the front discs until the rear shoes have had time to close the gap of 1/16th of an inch or so and contact the drums.

- Eric

 

[quote=MDchanic;277424]
The valve serves to equalize front and rear braking by delaying the application of the front discs until the rear shoes have had time to close the gap of 1/16th of an inch or so and contact the drums.

- Eric[/quote

are you sure?
i always thought is was to give more power to the front brakes because they do most of the braking. i thought that the back brakes would lock up faster with equal brake distribution because the back was lighter than the front, and even more so under braking.

 

These hold off valves were used on 1967-70 disc cars and changed just about every year. Starting on the left is the 67-68 valve ,the middle valve is the 69 valve, and the right valve is off the 70 cars. Notice the fittings change on the valves so they will not work with the brake lines from different year cars. It is important to note these valve are not available new, the ones with the large nut can be rebuild but it often costs more than going to the later 71-72 valve that Inline Tube offers new.

In 1969-70 a third valve was added to correct problems from the previous years. This valve now went in the rear line to restrict pressure to the rear because rear drums were now locking up before the front brakes were even activated. By 1971 GM had all the problems worked out and combined all these valves into one valve that is now called a proportioning valve.

From Inlinetube.com

 

To keep things interesting, in 1970, GM used both a "metering valve" and a "balance valve" in disc brake cars.

The metering valve is what I have been calling a "hold-off valve." It goes in the line to the front brakes and delays front brake application slightly (based on pressure) to allow the rear shoes to seat before the front discs bite. It has a button on it to press to make bleeding the front brakes easier.

The balance valve "is designed to proportion the hydraulic pressure to the rear wheels when more then 310psi is required at the front wheels on E-body, and when more than 530psi is required on B/C-body."

So everybody's right...

- Eric

edit: you will note that the balance valve was not used on A-bodies.

 

thanks for the info. i didn't know that. i have an NOS valve that goes by the master cylinder but i have no idea what year or model it fits.

 

Thanks guys for all the great info and thorough answers!