[mgchevyparts]

The 1 1/4" master is used with larger piston calipers, if you use it chances are it'll be too firm. True smaller bore (master) equates to higher pressure but it also means less volume. People automatically think higher pressure is better, the old "bigger is better" syndrome. Higher pressure is not necessarily better, what is better is MATCHING master bore size to caliper piston size. Smaller caliper piston(s) usually means smaller master size. Most D52 calipers, as used on 1/2 tons have a 2 15/16" piston which likes a 1" bore master for manual and a 1 1/8" bore for power, but small up/down changes can be made to fine tune the firmness/feel/pedal travel.
If I were you I would buy the correct 1 1/8" master