Our D4c Has been a never ending source of entertainment lately. The dozer functions all go into bybass when actuated during warm up.If I barely open a valve it will move the hydraulics very slowly ,but If I open them further it goes to bypass. By the way it doesn't seem to affect the ripper . After it gets warmed up it operates normally.
Need to put a pressure gauge on it and see what's going on sounds like a relief valve problem odd the ripper works tho.
its a very simple circuit, all I can say is check the main relief valve. post the serial number se we can double check for anything that may cause your problem