Hy tomc9770, welcome, as a general rule I would not install a master cylinder with a bore smaller than the original master cylinder, as you did not tell us what series/year truck you are working with we can only guess what size bore your truck originally came with. One note of caution with the brake kits, make sure the master cylinder you purchase is for the type of brakes you have, drum/drum for drum brakes, disc/drum for a disc drum setup, and disc/disc for an all disc brake setup, any system with the same type of brake at all wheels should not require a compensating valve, hope that helps.
