Master cylinder for rear disc

[Streetbu]

For stock front brakes and F body rear 12", yes usually.

 

[oldsmobile joe]

There is no valve in the master cylinder.

True

What you are referring to is a residual pressure valve inside the disc/drum proportioning valve, which you've already taken care with the disc/disc valve.

This is an internet urban myth. Our generation of cars never came with one. I've taken them apart, it doesn't exist.
If necessary, you can have rear discs with the stock valve

 

[81cutlass]

I have rear disks with the stock valve and the brakes work well with the late 80's B body wagon master.

Stock master doesn't have three bore diameter required to actuate rear disks.

 

[scoti]

I have rear disks with the stock valve and the brakes work well with the late 80's B body wagon master.

Stock master doesn't have three bore diameter required to actuate rear disks.

Could you elaborate on this statement?

 

[81cutlass]

Could you elaborate on this statement?

The not three, darn autocorrect...

But what I mean is that the bore on the stock master doesn't have the volume required (in my experience) to actuate the rear disk calipers. The rear disk calipers have a larger volume requirement than the drum cylinders and I experienced poor pedal feel, the pedal went to the floor and never firmed up despite the lines being bled well. The stock master is stepped and supplies more fluid to the front disks than the rear drums, going to the B body wagon part, even though it was built for rear drums, has enough flow. I can only assume the B body wagons used larger diameter rear wheel cylinders or GM decided they needed a larger bore master for the heavier car.