756 Hydraulic Issues

We had one of our 2/10's rain days today so I pulled my progect 756 in the shop to fix the last issue "hopefully" before paint. The rear pump is sucking air. I hooked up a flowrater and could not get a GPM rating as it seems to be flowing 50% air. The three point hitch also raises in jerks and is spongy with no load. I pulled the pump for inspection. The seal on the suction tube to filter housing was gone so I replaced that and the oring reinstalled and filled with oil. Same thing, sucks air. The front MCV pump has no issues, no air burps, priming issues, nothing. So I think the oring between the speed and range gear housing must be good. The only other place it could be sucking air is the pickup tube I guess. Looking at it from pump hole I could see no cracks and there is no movment in its mounts or frame. Were do these things crack or does the oring just leak? Its hard to belive an oring is letting in all this air unless it totally gone. Also if its sucking all this air thru the pickup tube I would think the front pump would be getting some air too. I can't think of were else this air could be comming from, any ideas? Anyone changed a pickup tube without pulling the cover? I am out of Hytran so I guess I'll have to get a pail monday and try the 5 gallon overfill thing and see if that solves the problem.

I'm leaning towards a bad pump. If you were picking up air you should still get a flowmeter reading. Every thing else would show up as a common problem with both pumps. Take the three point upper link cover off and start the tractor and see if you can see a whirlpool in the HY-Tran where it's prawing air down to the pump.
Chuck

I have the cover off. There is a definate sucking noise somwhere's in the front of the case. Hard to tell tho the drive gear is throwing and churning the oil. The oil dropping down from the top is churned to almost foam so the oil turns white pretty fast. I can dead head the system with the flowrater and the relief valve releases at about 1500. I can only get about 5 GPM at 700 psi at pto speed with all the air going thru. I had the pump out and looked it over, no ovious damage and it held oil while sitting on the bench. Some goober has had it out before tho because the drive gear nut is welded to the gear and shaft. Would it be sucking air thru the shaft seal? The pump is under a lot of oil for it to suck air down thru the oil. Hate to be like Allan and drop $500 in a new pump and gear that's not needed but its looking like it has to be the pump.

When you say seal on suction tube, you do mean the seal that is driven into rear frame and not actually on the tube don't you. And the seal must be put in from filter side with lip toward center of tractor. Install tube after pump is lightly bolted down, do so from filter side . I'm thinking you probably did this but just had to throw this in. With out knowing any history on this tractor, does the pump have a closed in rear or cover on it if open. Some one could have installed a replacement pump and not put the cover on it .

Yes, the driven in one. I installed it lip in then bolted in pump then installed tube from filter side into pump. The pump is the closed in type, Cessna, no plate. When I had it on the bench, I covered the suction port with my palm and spun the gear. It sucked in my palm. I realize this means little but at least a seal is not completely gone. I'am going to get some more oil tomorrow to cover the suction pipe just to rule that out. I guess if that dose'nt work I'll replace the pump. If that does not work I'am at a loss as to were the air could be comming from, its really a simple system.

Sounds like you have done a good job of trouble shooting and if you replace pump without success I don't have an answer right off the top of my head.