JD 450 B Hydraulic ( HELP NEEDED)

[Mr. Luke]

JD 450B track loader, Hydraulic problem. When I start the dozer the hydraulics are fine until I go above the half way point on my throttle. Once I go past 1/2 throttle and use a lift funtion once and release a clunk sound is heard at the valve and tank area and then the pump becomes starved for oil and whines. By just lowering the throttle has no effect I have to actually shut the dozer down and stop pump spinning and fire tractor up again. At full throttle the pump is putting out 2350 psi to the valve. I can drive the tractor at this speed ok and push ok but when the lift function is used the slightest, it seems to pop a valve and shuts inlet fluid to the pump. I 've replaced oil (aw 32) replaced filter and cheched sceen in tank, I even removed the tank screen and hade same effect.
Anyone have a clue what I am up against. Service manual don't cover this one. Or else i missed a step. Having to run this thing at half rpm is really driving me insane. I still have plenty of snow to move here till April.

[srs_mn]

Does the same thing happen while using the bucket curl or dump function at full throttle? If so, it could be a collapsing, or restricted, suction line between the reservoir and the pump.
If it ONLY happens to the boom lift circuit, then it is probably an internal valve problem... (I suspect the lift check ball). These JD valves are much more complicated than your run-of-the-mill farm store hydraulic valves - they have internal spring loaded lift checks, and built in relief circuits, that are spring loaded... I suspect that a spring or something has failed and is blocking the ciruit when the oil flow gets high enough to move things around in there.
Your John Deere Dealer should have repair kits available... get a schematic drawing of the valve or, at the very least, a parts book, to show you how things should look inside.
Good luck.
srs/MN

[digitup]

Mr Luke try your valve bank get a J D service rep to look at this and dont do this your self I have owned lots of Deeres over the years and three times had valve plungers brake in the Mark the hoses before taking it off. Digitup.