Bleedin’ hydraulics!

[Kiwi BlokeParticipant @kiwibloke62605]

Having done some work on the hydraulics of my tractor and log lifter/splitter, I’m puzzled by the complete lack of any provision for bleeding air out of the systems. I’ve mentioned this to a couple of tractor mechanics, but get blank looks: either it’s never occurred to them, or the answer’s so obvious that I must surely be an idiot. The nearest I got to an answer is that air, but not fluid, will pass the pressure seals, so the system bleeds itself. I find that hard to believe. Even if it is the answer, one would need to have a cylinder orientated with its pressure seal high, so the air was against the seal, but of course that may not be feasible.

Whatever the answer, I was pleased to find that the front-end loader worked fine, with no evidence of ‘bounce’, even after having had to reinstall the cylinders nearly full of air. However, one of the the log lifter’s pressure relief valves squeals a bit. Might this be because the cylinders and pipes are now full of air/oil ‘cream’? (Distant memories of Citroen warning about never pressurising the oleo-pneumatic system if there were any air in it. I believe it took ages for the air and oil to separate).

 

[renardiere7Participant @renardiere7]

Most tractor and excavator hydraulic systems I’ve worked on self bleed. After having the system open the rams are cycled in and out slowly building to full out and full in. I’ve always assumed this just slowly pushes any trapped air into the tank to accumulate there. In fact on my current excavator there is a procedure to follow to pressurise the hydraulic tank with air following oil top ups etc.

[PeteParticipant @pete41194]

I’ve operated hydraulic equipment over most of my life, and none of it needed more than a few cycles on the rams to push the air back to the hydraulic tank. So those systems would self bleed any air out. However that’s for large to very large industrial machines. If its a small and light weight almost consumer grade of tractor. I can’t say all or any brands of those are designed to and would do the same. Logically they should, but I have no experience with agricultural equipment.

[noel shelleyParticipant @noelshelley55608]

Having worked on any hydraulic system make sure that the reservoir is full. Then cycle the system several times fully to expel the air. Now check the reservoir again and top up if need be. If the system has over centre valves in it, it may take many operations over days to get all the air out.

I ve never heard the one about air bleeding past the seals. Depending on the tractor and system, open centre and closed centre are very different systems and are not interchangeable. When bleeding if you have hose failure valves you may have to bleed at hose fittings to stop the valve locking.  Good luck.  Noel.

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