maybe the replacement pump has some bad or missing internal parts?
I'm Not Sure, The original problem when I bought it was "no hydraulics at idle, and they began working over 1000-1200 rpms. I did not own the tractor that long and since this was my first tractor with loader hydraulics, I called my local dealer to service. Technician came to my farm and did a complete service. Changed engine oil and filters, Hydraulic oil and filters, Fuel filters, fan belts etc. When Changing the hydraulic oil he mentioned that it was extremely black, and stated that it could have been under heavy load for a while and never changed. I didn't think anything of it. When he put new oil in it was foaming so bad that it was pouring out the dipstick. He put a gauge in the rear accessory attachment and checked pressure. it read 2500-2600 PSI, he stated to me that it should be 3500 and that the pump was bad and cavitation was causing all the foam. No further diagnosis on his part, the service manager scheduled a pickup to change pump.
You can already see why I'm skeptical, Technician told me 3500 psi and when i checked the manual it is a 2500 PSI pump.
When I got the tractor back 2 weeks later, I blew 3 hydraulic hoses on the frontend loader in first hour of operation over a 2 day weekend period. When I called the service dept to explain that i thought something else was wrong, they told me that the hoses were old and that the new pump was probably finding weak points. I replaced every hose on the loader (mind you, the oem hoses were rated for 3000 PSI). The foaming never went away, I have been battling this "air" in hydraulics since late January. Pump was changed in December but I did not have time to even start the tractor until later in January.
I replaced all orings on suction pipe (5 total), changed hydro filters again, and on and on.
It appears as though the engine oil mixing could have been causing the hydro problem all along?
I'm not sure if it matters, but the previous owner said he had trouble with hydraulics right after he bought it like within the first week of operation, and his dealer put "new seals in the pump" and he said it has been fine ever since, that was back in 2008. He was not exactly sure what was changed out.
Sorry for the long post, I'm just trying to give as much info as I can.
I'm really skeptical of dealers technicians ability to troubleshoot and do not want to go down the $$ rabbit hole of changing parts, especially at a dealer rate.
I have been around engines, transmissions, motorcycles, ATV's, etc my whole life. I have rebuilt several gasoline engines, I try to be very humble and ask questions when I encounter something new such as diesels and hydraulics, I have basic knowledge of hydraulics and even less about diesel engines and am by no means an expert.
So in summary,
A machine with 700 hrs has had
1 New pump at factory (0 hrs)
2 New pump from factory rebuilt with seals (0-10hrs)
3 A replaced pump New at 709 hrs
4 a bad replaced pump. (currently)
seems to me something is being missed