The hoist pump on my trailer quit pumping. I have a connector to operate the hoist from my tractor hydraulics, and it still works with power from the tractor, so that narrows the problem to the pump/valve assembly. It quit suddenly - I hauled four loads of gravel and it dumped them just fine, but refused to work on load five. Everything looks normal, except for some air bubbles escaping around the valve spool. Is this thing repairable, and are parts available? I haven't cleaned it up for name and number yet, but figured some of the old truck guys on here would recognize it and be familiar with it from past experience. Thanks for any tips.