Hi,help needed  I have a Transcrete P35 and it’s developed a slow stoking piston on one side, it gets worse the more pressure required, I’ve had rod seals replaced with no effect, any thoughts pls ???? pump is 12 years old 

Update for anyone experiencing same symptoms 

first had piston rod end seals replaced made no difference 

another mechanic removed all the cartridges form spools and found damaged and missing o rings, replaced them and now my baby pumps like new again ????