hi don't know if you can help I run a 26x4 scwhing 1994, just lately when you turn on pump rock valve, will stay where it is ram will go up and back down then rock will change over , but wont go over  all the way, and on the first stroke, it labours a bit, would the seals need changing on the end of the main pumping rams , gland seals was done two years ago any help would be great thanks

you are most likely dealing with some stuck spools in the system, whats the condition of your oil? And given the age of the pump, some worn out components 

hi all sorted today changed, check valves  there are four, put three new ones on , and now good as gold