What is the purpose of the valve right below the master cylinder on a 1970 W-30 auto,with power front disc brakes. It has a black rubber boot on one end and line going from front resivour of the master cylinder. is there some kind of button under that rubber boot and what position is it suppose to be in ? Feels like mine is pushing outward.

 

From the internet....

"A brake metering valve is used to Balance the Braking Forces. It is only found on vehicles with front disc and rear drum brakes. The metering valve holds back fluid to the front brakes until enough pressure is created to overcome the return springs inside the rear drums. It is also known as a “metering block.”"

Yes, there is a "button" under that rubber boot. The Chassis Service Manual shows that it gets depressed as part of the brake bleeding operation....apparently to allow full flow/pressure of the fluid to the front brakes while bleeding...thus speeding up the front brake bleeding time.

The button being in the "out" position is the correct position for it (other than when bleeding). There was a special tool made to clamp over the valve and hold that button in while bleeding the fronts.

 

thanks. Makes some sense now. My pedal is very hard and just trying to figure out why.

 

I don't know about the interwebs, but Olds never called the metering valve a "metering block". They did sometimes call it a hold-off valve, which is a more descriptive name. The drum rear brakes will always have some 'slop" in the linkage that pushes the shoes out against the drums, especially if the brakes are not properly adjusted. The metering valve slightly delays application of the front disc brakes to allow the rears to "catch up". Contrary to what a brand-name brake parts supplier claims, this is NOT a proportioning valve. Olds did not use a prop valve on the 1967-70 cars with disc brakes, they balanced the front/rear brake force by changing the bore on the rear wheel cylinders. As for bleeding the system, in half a century of doing this, I've never depressed that pin (which is usually rusted in place anyway). I suspect this may be simply to preclude a pressure differential in the distribution block that could push the internal piston to one side and trigger the BRAKE light on the dash. I use a vacuum bleeder system that won't do this anyway.