How do I bleed the air out of my hydraulics?

STx · Oct 27, 2015

I know this is a pretty stupid question but... I have NH TC40DA that had a lower rubber fitting get damaged and I lost about 2.5 gallons of hydraulic fluid during the replacement. I've fixed everything and topped off the sump but I know I have some air in the system. When I start the tractor cold, I get no lift on the bucket or 3 point, if I put the throttle up to about 1200RPM, it'll build some pressure and thinks will lift but jerkily. If I put the hydro transmission in gear and let sit for a minute, it builds pressure and things lift pretty smoothly but a little slowly. After driving it around a bit, the lifts get smooth and back to normal speed. I thought that maybe just letting it sit would allow the air to work its way out but it hasn't happened that way.

I have the service manual which tells me how to completely rebuild every single part of the hydraulics but does not tell me how to bleed the air out so any direction you guys can offer would be greatly appreciated. It "works" with the air in it but I'd sure rather have it back the way it should be and one day I'm going to have to change the fluid completely so knowing how to bleed it will come in really handy then as well.

Thanks!

newbury · Oct 27, 2015

I've always just worked all the cylinders.

STx · Oct 27, 2015

I've done a lot of working the cylinders to full extension and retraction several times, I've put about 9 hours on the machine since I fixed the leak and refilled the fluid but I still have the issues when it's not been run for a day or so.

Maybe I have a leak somewhere but I don't see any fluid under the tractor or on anything.

Wagtail · Oct 27, 2015

STx said:
I know this is a pretty stupid question but...

Not a 'stupid question' at all, Mate. I haven't encountered this problem and wouldn't know the procedure either.

This is a knowledge sharing forum, after all.

Harry in Ky · Oct 27, 2015

The system will bleed itself eventually, but only once you've eliminated any sources for air. If you had some lines/connections opened on the suction side of the pump and suddenly NOW have air in oil, then that would be a likely starting point. Did you re use the original hose clamps? In my opinion the OEM hose clamps are always suspect, even with new equipment. Once loosened and re tightened they become even more so. I generally don't put them back once removed. I like to go back with better quality clamps and tighten them with a 1/4 drive ratchet, not just a screwdriver where suction lines are involved. You mentioned something was "damaged". Did you follow the piping in both directions looking for any other connections that may also be involved now that weren't before the damage occurred?

STx · Oct 27, 2015

That's what I thought initially, that it would self-bleed but, I would have expected it to have done that by now. The part I had to replace is the one the red arrow is pointing to, it's a rubber fitting connecting 3 hard lines together. I don't think this is a suction area, the rubber fitting had a small tear in it and was spraying fluid at pressure when the tractor was running.

I did use the OEM hose clamps and did tighten them with a 1/4" ratchet, they can not go any tighter and did not break when tightening. I think these are actually pretty good quality clamps, I cranked them down past the point that most hose clamps would break.

The problem was initially caused because the bolt holding the P clamp that keeps the front rigid line vibrated out (I learn about more bolts to check every time I run this tractor) and then a limb got wedged in there which pulled the rubber fitting off and also put a small tear in it. I've fixed all that and have a hose clamp holding that rigid line in place now until I get the replacement P clamp, I don't think it's pulling any air in there and it's definitely not leaking there anymore. I do have a small deformation in the rigid line that runs up but it's above where the fitting is clamped on.

I haven't fully followed the lines to see if there were problems on the other end, I'll take a look at that and see if there is anything going on there.

Thanks for the feedback so far.

Geoffff · Oct 27, 2015

STx said:
I know this is a pretty stupid question but... I have NH TC40DA that had a lower rubber fitting get damaged and I lost about 2.5 gallons of hydraulic fluid during the replacement. I've fixed everything and topped off the sump but I know I have some air in the system. When I start the tractor cold, I get no lift on the bucket or 3 point, if I put the throttle up to about 1200RPM, it'll build some pressure and thinks will lift but jerkily. If I put the hydro transmission in gear and let sit for a minute, it builds pressure and things lift pretty smoothly but a little slowly. After driving it around a bit, the lifts get smooth and back to normal speed. I thought that maybe just letting it sit would allow the air to work its way out but it hasn't happened that way.

I have the service manual which tells me how to completely rebuild every single part of the hydraulics but does not tell me how to bleed the air out so any direction you guys can offer would be greatly appreciated. It "works" with the air in it but I'd sure rather have it back the way it should be and one day I'm going to have to change the fluid completely so knowing how to bleed it will come in really handy then as well.

Thanks!

Im scratching my head over the very same question ( how to bleed hydraulics ) Iv been searching the site for hours and only come up with contradictory advise leaving me even more confused then when I started :confused2:

http://www.tractorbynet.com/forums/yanmar/130855-how-bleed-hydraulics-2.html
post # 19 says you don't bleed hyd.

Harry in Ky · Oct 27, 2015

Have you actually examined a sample of fluid when this condition is present? Does it appear foamy and full of bubbles? Does that more or less disappear when the hydraulics finally settle down and go to work? IF, and I'm just guessing at this based on your descriptions and symptoms, your pump is sucking air, it's probably not coming from the area in your diagram. That plumbing is related to charge pressure and/or return flow to/from the hydrostat. The areas of concern would be the inlet tube before the hydraulic filter base, the filter itself(needs changing or possibly just loose), the tube from filter base going toward pump, the rubber connection between the suction tube sections, or the seal at the base of the pump inlet. There is also a suction screen on the inside, but you have to drain the rear end to access that. The pump shaft seals could be involved, but the other items should be explored before going into the pump.

STx · Oct 27, 2015

I have looked at it and it doesn't look milky/aerated. The problems do disappear once everything has been running for 10 minutes or so, FEL operation is smooth (although a little slow but not drastically so) and the 3 point lift holds just fine. I do have a small amount of surging on the 3 point when fully lifted but this was happening before I damaged the fitting.

The fluid and filter were both changed about 170 hours ago so they're still pretty new. The fluid that drained out was still clean and I don't believe it had an opportunity to suck up anything when it broke.

I'll give it a more thorough visual inspection tomorrow and see if I can find anything that looks wrong.