Golden Jubilee hydraulics not staying primed

I have my dad's 1953 jubilee which he bought when I was 7 years old. I remember the day it was delivered. Very good condition for this model. Lately I have had some problems with the lift not working after sitting for several minutes. The piston pump seems to loose its prime. I can bleed just a very small amount (attached photo) of oil/air out of pump and it will lift until it sets for awhile. Sometimes only 30 minutes. Then I have to bleed it again to get it to lift. Anyone know why it losses prime in such a short time but it only takes just a small amount of bleeding to start it again?

Thanks for any info
Cgun

Tell us more. Is that a new aftermarket pump?
If so, did you change all the o-rings on the tubes when you
put it on? What did it do before the pump was replaced?

Are those washers? between the tube flange and the pump?
Probably where I would look first.

From NAA experience. Tubes get bent, but, the flanges on each end must fit square with new/good o-rings.
(last NAA pump I changed, the tubes/flange went 'sproing' when I removed the bolts/studs......didn't even come close to lining up afterwards. Tubes were bent and under tension.
Took some time to re-align...those tubes are tough...but the flanges must fit square, or the 0-rings will fail)

If all that checks out, early signs of pump failing...is loosing prime.
If that is a new/rebuilt pump, internal leakage is possible if the rebuild wasn't done right/completely.

Do a search for posts by Kurt-NEPA.
lots of pump rebuild info.
Sorry to see most of his pics are gone because of the photobucket debacle.

This is the original pump (as for as I know) that was cleaned and rebuilt by a local mechanic.

Last year when I tried to pull a concrete culvert out of ground thr lift stopped (overloaded) working. I had a part time mechanic look at it and he rebuilt the pump and changed the fluid.

It worked for awhile (month) then it needed priming after sitting overnight. Would work ok until next time it sat.

A few months ago it started loosing prime after sitting for a hour. I pulled the pump and found the front gasket (pump end) was torn. Also the plate that is next to the gasket with the matching slot was not inline with the outlet. It had been flipped to get all the holes inline.

I corrected this and got new gaskets and manifold (pump end) o-rings. The new o'rings were not much thicker than the groove in the plate (maybe a few thousandth).

I put the pump back on but it still looses prime after sitting a short time.

When I bleed the pump air bubbles continue to come out even after it runs 2-3 minutes with the clear tube going back to the fill port.

What confuses me is that when it will not lift I can open the pump port and close it when the oil/air starts to flow into the tube and then pull the control up and the lift works.

I am sure all the air is not out but seems it only takes "cracking" the outlet to get it started.

By the way the washers on the port are to keep an o'rings compressed which I added to assure I have an air tight seal under the outlet.

This is the first time I have ever attempted to work on the hydraulics so I am not familiar with the operation other than what I have read on internet posts.

Any help is appreciated. Thanks