69/70 B body hold off valve, bleeding question.

cdnpont · Canadian Poncho Superstar!

69/70 B body hold off valve, bleeding question.

seventy2plus2 · Canadian Poncho Superstar!

If I recall, the service manual indicates to push the button on the valve when bleeding the front brakes. It only moves about 1/8".

cdnpont · Canadian Poncho Superstar!

Thanks Clint, this is exactly what I'm looking for! Need to open my eyes!

I think I have the 70 version of the valve? (no nut on the end).

And just as you suggest, there it is, described in the 1970 manual. Section 5-5, Manual bleeding, 8, Note;

And further in the 1970, 5-21, Disc Brakes, Bleeding system. Describes the 1/16" motion.

The valve I have was rebuilt, and moves freely which is good.

It's also in the 69 manual, 5-20, Maint and adjustments, Bleeding system.

seventy2plus2 · Canadian Poncho Superstar!

I'd wager no one presses the button when bleeding the brakes.

cdnpont · Canadian Poncho Superstar!

I wonder if it would completely restrict the flow of fluid at gravity or low pressure?

Is the rubber cap easy to remove. I'd prefer it not be on if I gently clamp the valve open. Don't want to wreck that little original cap.

seventy2plus2 · Canadian Poncho Superstar!

cdnpont wrote:
I wonder if it would completely restrict the flow of fluid at gravity or low pressure?
Is the rubber cap easy to remove. I'd prefer it not be on if I gently clamp the valve open. Don't want to wreck that little original cap.

I haven't had one apart in years. I sent you some documents re the 1970 valve.

-- Edited by seventy2plus2 on Sunday 28th of April 2024 01:10:38 AM

pontiax · Poncho Master!

My 2 cents, when i bled my disc/ drum setup. I had to use a tool in the proportioning valve to allow the front to be bled.it fits in place of the switch.

cdnpont · Canadian Poncho Superstar!

Thanks Pete. I only have the 70 stock Distribution block that you circled, and will be adding this 69/70 featured, stand alone pressure metering valve (Aka, hold off valve).

It holds off the front discs until the rear drums play catch up so to speak.

In 1971 GM introduced the Combination valve, which combined the metering function of the 69/70 stand alone, with the brake pressure differential (waring light) and pressure isolation switch found in the old distribution block, and a new proportioning function that cuts the rears back proportionally in a heavy panic stop.

I've heard you need a special blocking tool that threads into the sender switch when bleeding a Combination valve as it trips and must be reset. I was frustrated years ago by this, for lack of that knowledge.

But now I'm thinking, have you had trouble bleeding the old Distribution block system? I've never had one trip and not return.

gparis7 · Canadian Poncho Superstar!

On the factory 69-70 units, all I have ever done is hold in the button while bleeding. The brakes always worked well on the four cars that I have or had with the single piston factory disc brake setup.