Bubbles in hydraulic reservoir.

   / Bubbles in hydraulic reservoir. #1  


Bronze Member
Oct 26, 2013
Benton MS
3414 IH
Hey guys, I’ve got a farmpro 2420. I started having overflow out of my vent tube. (Problem)lol

Please read below before rushing to give advice. Likely already checked the basics.

Well after inspecting, it appears I have air feeding into the system. Let me explain.
On this tractor there is only a hard line going front the reservoir/filter to the hydraulic gear pump.

From there it goes to a diverter valve, then the steering, to the back lift.

I’ve checked thoroughly. No gunk no obstruction to cause cavitation in the sump. Not a loose filter assembly. Not a hole in the hard line. Nada! Got it back to the pump and all lines are tight and no sign of leaks or loose fitting.

This just spontaneously started. The only thing I feel I noticed is when it happened my hydraulics slowed down.

Watching through the fill port I can see minor fizz when I first crank it. After it runs a while it gets so agitated it boils out. It appears that using the lift slows or calms the agitation.

No the fluid level isn’t low causing the foaming. No it isn’t high or overfilled. I’ve checked cold and when it begins to run. No possible way to check when hot unless I drain it dangerously low. Yet again it’s not a true reading because of the foaming. The lift still works but has minor signs of the air in the system. Like a slight jerk when first engaged. Perceived minor slowness in lift speed.

To my understanding on the outlet or output side it would simply manifest as a drip “leak” due to the pressure pushing the fluid “out”. On the suction or inlet side it can suck air “into” the system! Thus the foaming produced through the system.

Other than the gear pump itself somehow sucking enough air. THATS A LOT OF AIR.

Any ideas, seasoned wisdom or tricks to pinpoint this problem???
   / Bubbles in hydraulic reservoir. #2  
You are correct in thinking it is probably a suction side issue.
All of the seals in the system get old and brittle with time and use. Could be an o-ring at either end of the hard line or where the adapter mounts to the pump. The pump itself has seals between the body and the end plates that can go bad too.
Have you taken the suction line off at the pump to see if fluid flows freely at that point?
Lack of flow could possibly cause vaporization (cavitation) at the pump rotors, I think anyway. One would think that it would cool and condense back into liquid before it returned to the tank but I'm not sure about that. Bob Rooks, what do you think?
BTW, what kind of fluid are you using and how old is it?
   / Bubbles in hydraulic reservoir.
  • Thread Starter
Well how this works is as follows...

In the sump or reservoir there is a brass mesh filter with a pickup tube.

That runs vertical and connects to the outlet or bolt on bracket which has the outlet for the hard line.

Now the hard line is a mushroom taper fitting. No O-rings.

It connects to that outlet and runs from the sump all the way to the front of the engine which connects to the hydraulic pump. No O-rings in the circuit.

I have disassembled this gear pump months ago during an engine rebuild in order to verify its condition. I was hoping to get advice first before tearing in that far.

At that time everything looked new inside this pump. As for now I’ve only inspected up to the pump but not tore the pump back apart.

As for the fluid I use universal hydraulic fluid. It’s new looking fluid. Slightly reddish but still clear. I don’t suspect that as it’s been running fine on the type and was used prior to me is Owning the tractor.

I just wanted to make sure this couldn't be on the output side somehow. Or some other tractor mojo that I’m not understanding. No matter how I tried I couldn’t get that idea of the output to pass muster in my brain.

It has to be on the suction side or the pump sucking air somehow!

Maybe a seasoned ole goat will chime in and let me know if I’m right or horribly wrong.

I thank you for the reply!! Any further advice or help is greatly welcome!
Last edited:
   / Bubbles in hydraulic reservoir.
  • Thread Starter
Oh and it flows fine. Me being an idiot took off an output line and put it in a jug. ““Testing your exact theory””
I was just going to bump it over to see the flow.

I seen it alright! The motor starts on on a half a bump!! 🤣

I got drenched before I could shut it down!

Yeah that hard line is clear.
   / Bubbles in hydraulic reservoir. #5  
Foaming can be caused by water in the case/reservoir too. When was it and the filter changed?
   / Bubbles in hydraulic reservoir. #6  
Remember that air can only come in at the suction side and that includes half of the pump.
So ...remove the pump and replace that shaftseal. Replace and test it !
The rubber of that old seal will brobebly be very hard and will brake if you press on it.
   / Bubbles in hydraulic reservoir. #7  
That sounds almost exactly like my Jinma 284. On mine there is a 18X2.65 o-ring where the strainer tube screws into the bottom of the 3 bolt adapter plate on the top of the reservoir. This is inside the tank but above the fluid line. If that dip tube going down to the brass screen is loose or the o-ring bad, air can get in there.

Where the suction tube connects to the side of the pump there is an adapter held to the pump with two bolts. There is an o-ring between the adapter and the pump, another place where air can get in.

After that about the only place left for air to get in would be at the "W" seals under the back plate of the pump.

I've never had my pump apart, but have rebuilt a backhoe pump which is very similar except the pressure port is on the side instead of the back end. On that pump the only shaft seal is on the PTO input shaft on the front end, the back end holds the gear shafts and then seals against the housing. I believe on the engine driven pump the input shaft seal is on the pressure side of things as there have been many cases of this seal failing and the pump putting all the hydraulic fluid into the engine crankcase.

Good hydraulic fluid will keep small amounts of water suspended in fluid and usually develops a "milky" appearance if there is water in it. If there is too much water for the fluid to hold it will drop out and collect in the bottom of the sump. If you can get a sample of the foamy fluid drawn out and into a clear container, set it up where you can observe it for a day or two. If it is air the bubbles will eventually rise to the top and the fluid will clear, if it is water it will stay milky looking or drop to the bottom if there is a lot of it.

I suppose if there could be a hairline crack in the metal suction tube, most likely under one of the connecting nuts. It might look fine when the nut is off but when tightened down, it could open up enough to let in air. I have seen this kind of thing on soft copper pipe, steel brake lines and JIC hydraulic fittings. Flare fitting leaks can be a bugger to diagnose but at least when on the pressure side you can tell which location is the problem. Also any debris or deformity of the flair mating surfaces can be a problem.

Sounds like you are on the right track. I would start at the suction screen and re-inspect everything going forward to the pump because doing that twice is easier than tearing into the pump. Those pump seals can be finicky and I hope you don't have to go that far.

Let us know what you find
Last edited:
   / Bubbles in hydraulic reservoir.
  • Thread Starter

Well.... what in the hell goes in front of the seal to keep it in place if it’s possible to push out?

Why did it push out?
Why is it sucking air instead of pushing fluid out into the crank case?
Sucking air but blowing the seal out!

There is a groove but it doesn’t look like a C clip groove

   / Bubbles in hydraulic reservoir. #9  
Looks like it's missing the circlip to keep the seal in place.

On a gear pump, the inlet side of the pump provides lubrication oil for the bushings and shafts of the pump through some grooves machined on a couple different places. Therefore, being that the pump is creating suction to pull fluid from that side and with the seal out of place like that, it will pull air from the outside.

I would say the seal is good and it's just crooked on the bore, however, since you're at it, might as well just change the seal, put a circlip on and be done with it. It's cheap insurance.
   / Bubbles in hydraulic reservoir. #10  
While this doesn't depict your pump, it does show that some pumps do use a circlip. I wouldn't hesitate to install one. Your pump may be unique because of the end cap ports, but all the ones I've had experience with (including myself) have always blown on the pressure side and filled the crankcase with hydraulic oil, usually from deadheading.