Hard to say. It definitely needs more pressure than manual 9" drums, since I did that swap last year (of course, they stop much better, too). Discs always require more pressure than drums, because drum shoes are designed to be "self tightening" -- as they grab the drum, the rotation pulls them in tighter, increasing braking force without requiring as much additional pressure. Discs are strictly linear -- more pressure equals more braking. BTW, this is why disc/drum systems need a proportioning valve (it's not to "balance" front/rear braking). You can adjust the amount or pressure required by choosing a different disc brake master cylinder bore -- a smaller bore requires less pressure, by slightly increasing the pedal travel distance (like using a longer wrench to get more torque). Sizes available include 15/16, 7/8, 1, 1-1/32, 1-1/8 (not all will be listed for A-bodies). Most people like 15/16 for manual brakes -- this is "stock" for early 70s A-body *power* disc brakes. Manual brakes came with the 1-1/8 bore, which is pretty macho.
