Rich, I think your master is too small. I believe you should have an 1 1/4" or 1 3/8" bore. Don't be afraid to use a bore listed as a metric bore of 32-36mm. You need to be pushing more volume. I tried a lot of master's when making my disc brake conversion for these larger trucks. I settled on these numbers from Carquest...
-Booster 5471098 1994-1996 Chevy 2500 gas
-Master E151640 1996-2002 2500-3500 Express van
Hope that helps.
