Gut the valve on the frame rail and use a 79-81 Trans Am master or a 79-85 Eldorado, Riv or Toronado master cylinder with a stock dual diaphragm booster for a G body. You will find the Power Master will not supply enough assist with the rear discs once you add the master cylinder with the larger bore to the equation.
If you have problems with a vacuum conversion, then you have other problems such as a bad check valve or tapping for vacuum in the wrong spot. Its very rare that a dual diaphragm booster goes bad on these cars. You never hear the TTA guys complaining that they have problems and they have 4 wheel discs with vacuum assist from the factory AND there is nothing really different about their engine set up. IMHO, if anyone has problems there is either a mechanical fault OR it is in their mind.