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For a 70BB I have the valve on the rear frame but isn't there another valve thingamabob under the Master Cylinder? Yes, I took it apart years ago and don't have it anymore; just have my new MC and Booster. Thanks....
 

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Yes the front valve is the disk hold off and the rear is a prop valve.

On my 69, I removed the hold off valve because it was sucking air. I just put a fitting in its place, and the brakes work great.
 

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I thought rear was hold off valve and font is proportioning vavle, at least that's the way I read it. I could be wrong..wouldn't be first time. Service manual calls the valve on top "metering valve" and controls pressure between front and rear brakes. Page 5-2 in the Chassis Service Manual 1970.
 

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Go to the mpbrakes.com web site(or google master power brakes). The tech reference section explains a metering valve (which you have) a proportioning valve, and a residual valve. Basically, all 3 functions were incorporated into a "combination valve" around 1973. Most people loosely and incorrectly refer to these combination valves as a proportioning valves. Great web site and products. They set me straight on this stuff by reading.
Most people dump the metering valve and install a newer combination valve to do all functions.
If you read your service manual carefully I think it will say the original master cylinders had residual valves built into them to hold some pressure against rear shoes. These built in residual valves were abandoned by GM when they introduced the combi valves. Know what year master cylinder you have to understand this issue also.
 

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From inlinetube.com...

1967-70 GM Hold Off Valve

Details:
This is the hold-off valve that mounts on the stud of the master cylinder on all factory 1967-70 disc brake cars. It is used in conjunction with the distribution block mounted on the frame, which is the same on factory disc and drum cars. This valve holds off the front brakes to give time for the rears to activate. At a calibrated pressure, the valve allows fluid to pass and engages the front brakes. This valve is the same on all GM cars, but the bracket is different for applications NOT listed.

The rear was a correction valve or rear hold off.


In 71 they started using the combination valve.
 
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