Unloading valve

[Gman]

Hello to all. As some on here know after my Power Major had sit for 10 years I was able to get her going again with the help of this site.
Not knowing too much about the hydraulics I cracked my pump on first use
and believe it was due to a stuck unloading valve. After reworking the valve chest the unloading valve worked like it should, opened up at 2000psi but now I don't think it is opening again and worry about the pump. There were two shims in the Power Major but do not remember if I took them out or not. I'm thinking about taking the one out of the Major I have for parts and see if it works better.
The pic shows part of the valve chest on the Major.
I'm correct that the unloading valve should be the same, Power Major is 1959 and Major is 57/58?
As always the knowledge on this site is greatly appreciated.

[whirly]

I have no expertise with this but .... The parts book says that the change in the "Valve (H.P.L valve gear unloading valve)" came in 8/58/60/11 - start & end dates for the PM. I'm assuming this indicates the PM had a different unload valve/pressure spring.



Best wait until Brian pipes up

[Gman]

Thanks whirly, I looked at the Major yesterday and valve chest looks the same. The seat is off the Major so I''m going to take the top off the unloading valve and see what it looks like.
Thanks again.

[Brian]

The change in the unload valve had nothing to do with pressure, that remained the same.

The valve was changed to improve leaking and wear that occurred and to help with re-seating faster after lifting off. The valves are not all the same and are not interchangeable unless you fit the whole valve chest although the later valve can be used in the early Power Major if changed as a unit and the shims are modified to fit in the bore.

[Gman]

Thanks Brian, when I first reworked the valve chest and unloading valve it released at 2000psi but as soon as I put my box blade on the unloading valve did not seem to release correctly. I have been watching the psi with the gauge I mounted and will back off with the control lever when psi reaches 2000, don't want to burst another pump so therefore my concern of getting it to work correctly. I'm correct that it should not matter that I have weight on the lift arms, unloading valve should still release at 2000psi. Thanks as always for your advice.
Gerald

[Brian]

That is correct, with load on the lift arms or just "dead heading" the valve should release at the same pressure.

Sometimes it is forgotten that the lift does not work at 2000 psi all the time, the pump pressure is only that required to lift the load on the arms. So at times, with a light load, the pump pressure might only be 500 psi when actually lifting.

[Gman]

Thanks Brian, your help is always appreciated. I guess I will pull the unloading valve out and see what is going on. I don't understand how the litte ball in the bottom stays in place if the valve lifts up off of it. I will use some chrome polish on the valve, maybe still a little rust I didn't get off the first time. I'm one that does not like making same mistake after the first time, well except for women , I don't seem to learn.
Good luck on all your projects.

[whirly]

I thought you might be interested in this Major/Power Major Supplement on the Hydraulic System. The first part explains the unload valve changes.

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