there isn't any adjustment on the valve block for the inch piston the only adjustment is by adjusting the linkage length not the master cylinder rod length but make sure the brakes are adjusted correctly before you adjust this. The inch should engage just as the brakes start to bite. One thing to check are the springs ok on the inch spool and the modulation spools??? I have had problems with them in the past also what oil is in the transmission lots of people put red Dexron 2 Atf oil in them it should have a JD40 spec oil which is normally brown The filler hole you found isn't a filler hole its a pressure test port