f700 dump trk hydraulics

[RustyR]

I have this old dump trk that has a pto driven gear pump that runs the hydraulics. What has happened three times now is that if I run the dump box all the way up, something happens to the pump or valves where it kills the engine. I can let the box down,will still kill the engine like it has pump has no nuetral anymore. Release the pto and it idles fine, engage it & motor dies. If I hold the dump lever forward, pump runs normally and box dumps again. I'ved cleaned the gears in the pump, always don't find anything but problem is gone for awhile again.
 

[Bill Lisbon]

You don't have a relief valve in your system, or if there is one it's not working. When your pump fills the cylinder there is no where for the oil to go so it stalls your engine, you are lucky, I've seen systems without a relief valve bottom out and split the pump wide open.
 

[RustyR]

I'll have to check over the relief valve situation. It used to be a city truck and still has valves and lines for a rear sander and ft blade. Something sticks under full pressure somehow.