The rock shafts on my 1948 G will not go down. I took the valve body off and it looks like the piston goes too far when it is raised. I put a 4' pipe wrench on the rock shaft and it did not go down. A small hit on the linkage with a brass hammer freed it up and it lowered. It is free to go up and down with a wrench. Does not seem to bind. I put it all back together and it went up fine, but is stuck up again. I can not move it to go down even with the large wrench. With the valve body off, I can tap the piston linkage and it is free again. I do not see a stop that would prevent the piston from traveling too far. Any ideas? Thanks, Paul