I'm doing a drum to disc swap on my 69' Chevelle. All I have left to do is the prop valve, master cylinder, and booster. Last night I was told (by the auto parts store) that the prop valve and booster on a power drum brake, or power front disc 69' Chevelle are the same, and the only part I need to change is the booster.

Is there any truth to this?

The booster is the same on a '69 power drum as power disc, however the master cyl is different. 1 1/8" bore MC for disc and 1" bore for drum (the MC is the same for manual and power drums though). The "proportion valve" that mounts just below the MC on the booster is actually a delay valve that delays pressure to the front discs til pressure has built enough for the rear shoes to be contacting the drums. It's not a real necessity since a car will stop just fine without one. I had a car without and later with one and couldn't tell the difference.

VOn is right on the money here. The 67-70 disc brake cars have ad elay valve that is mounted to the master cylinder and plumbed in line with the front brakes. As he said..not really necessary for functionality. The purpose of it is to prevent the car from nose diving under heavy braking. I ran my 70 for years without it then installed it just for the sake of looking correct.

yeah, it's more of a pain when it comes to bleeding the brakes if you try to use a vacuum bleeder by yourself, doesn't work well. The delay valve doesn't like a vacuum on from the wheel cylinders.

I'd use a combo valve before a delay valve if I had it to do over again.

yeah, it's more of a pain when it comes to bleeding the brakes if you try to use a vacuum bleeder by yourself, doesn't work well. The delay valve doesn't like a vacuum on from the wheel cylinders.

I'd use a combo valve before a delay valve if I had it to do over again.
You are keeping the plunger pushed in (on the backside of the valve covered by a rubber dome) while attempting to bleed aren't you? I use two tapered wood door shims wedged in between the booster and valve to keep the plunger depressed while bleeding.