A customer has a D-5H and is complaining of weak pushing, He has a service manual on the way. Just thought I'd get some barnstorming ideas til it gets here. Charge valve or something along that line? Obviously the first thing I will do is check the suction screen and filter and go from there.
First thing to do before even changing oils & filters is to check all the torque converter pressures IMO. Also has the problem slowly got worse or did it just appear overnight..? If it's the first it propbably points to wear (how many hours on the unit?) or if it's the second then maybe something has gone pop.
First you need to establish if it's a powertrain problem or an engine problem. I personally would start with a stall test.
I haven't seen the machine yet. Apparently this has slowly creeped up on them, When they have the manual I will be doing pressure checks and a stall test. Just getting ideas for now. And the issue is in all the shift positions forward and reverse.