Hello All,
I am very close to completing a drum to disc swap on my 69 Chevelle with the donor car being a 70 SS 454 Monte.

So far I have installed a PST front suspension kit, replaced the tierods, installed the coils, and mounted the spindles.

My car was originally a power drum brake vehicle, and I was told I could use the booster/master cylinder that was originally installed with the disc upgrade.

Yesterday I was told that I need to find a prop valve from the same year vehichle, and also change the booster/master cylinder. The monte is longer available to pull these parts.

Any input or advice will be greatly appreciated. I have to get this beast back on the road so it can drive me to my wedding. Time is running out.

Yes you need to change the master cyl to one for disc brakes. It has a bigger bore (1 1/8" vs 1" IIRC). No worries, you can get a rebuilt one cheap at the local auto parts store but many of those leak. You can also get a new one at the local store for a little more that has less chance of leaking. The proportion valve on a '69-'70 is actually a delay valve. It delays actuation of the front discs until enough pressure is applied by the master cyl to push the rear shoes out against the drums. My car didn't have one on it when I bought it and the brakes worked just fine. After I installed the valve I could tell no difference.