Hy Barnfind49, welcome, please remember to tell us the year and size of your truck. When you quote part numbers please tell us who's part numbers they are. If all you are going to do is upgrade to a dual master cylinder then do not go smaller than the original master cylinders bore, If you are planning a disc brake upgrade in the future then a different master cylinder should be installed now with residual pressure valves installed in the line to the front brakes which can be removed when you install the disc brakes, hope that helps.