U hydraulic relief valve

karl f · Apr 12, 2011

Is the hydraulic system on a UTU supposed to have a relief in the system?
I blew out the new square o-ring and gasket in the pump/hose flange accidentally.
oops
karl f

MMDEL · Apr 13, 2011

The relief valve is in the hyd box under the gas tank. Did the hyd pump sound like it was under pressure before it blew?? One way to check for your problem would be to remove the pressure hose from the hyd box add hose to it and run it back in the fill hole. If that works OK with out undue pressure then tee a press. gauge in the line and hook it back up to the hyd box static flow press should be next to nothing. Be sure the valves in the box are in neutral. If at one point and time someone hooked the hoses up backwards and the hyd cyl was mounted on the rockshaft it can force the lever to the box to strip the internal gear that moves the valve and lock it in closed position. This would put pressure on the pump at all times.
Maybe my rambling makes it more confusing?
MMDEL

Alan K · Apr 13, 2011

The relief valves are in the valve its self in the hyd reservoir. That happened on my ZA. I ended up using a round O-ring. It still got a small amount of wetness from oil by that spot but I dont think any more that it would have from new. I think it is kind of a weak spot especially when the cyl is under pressure.

karl f · Apr 13, 2011

pump was resealed recently and first time in 30 years it's been usable. saw the pressure hose at the pump jump when lever actuated against closed circuit. It was the visual and audible pressure test that blew it out in the first place. Every thing was fine before that. That's the rest of the story.
Noticed day after a big puddle of oil on the steering shaft below and an oily chunk of gasket sticking out of the triangle flange.

"Did I do that?"
karl f
Urkel's cousin?

MMDEL · Apr 14, 2011

Be glad it blew out your O ring and not split the pump body open. Did you use the square cut O ring or did you use a round one? If you can't find a square cut a quad O ring would be a better choice. If I follow your comments correctly, you said when you moved the lever from neutral things cut lose. Sounds like you are going to have to take your Hyd box apart and check things out. If it's been not used for 30 years, there may be something stuck.
MMDEL

karl f · Apr 14, 2011

MMDEL,
I used the square style supplied by Welters. It looks like a slice of rubber tubing. I had the reservoir cover off, but only to clean screen, inspect reservoir, and remove thin coating of sludge that was found in bottom. Lever parts moved freely before, during, and after disassembly and valve mechanism looked clean, so I decided to leave it alone. I'll replace the o-ring for now and hope it was installer error or material failure. I'll do actual pressure checks when I have the rest of the plumbing complete and can observe behavior with a cylinder.

karl f

Anonymous-0 · Apr 14, 2011

I got some of those square o-rings from Welters when the approx 20 year old repair on the ZB blew.

I went through 2 o-rings at the back of the pump in about 10 minutes of run time.

When I installed the second one, I could see the problem. The o-ring is sized to the Inside diameter of the bore. There is nearly 1/16" room around the outside of the o-ring to the side of the groove it fits in.

What happens is when you presurize the system, the o-ring stretches to the outside of the bore. And it only stretches that much a few times before it fails.

What I did to fix it was the same thing I did 20 years ago when I had no source for MM parts: I went to NAPA, and got a regular round o-ring that was a good fit in the square groove.

And its been like that for about 4 years or so now. Going on another 20 I hope.

Anonymous-0 · Apr 14, 2011

Sorry, I didn't really answer your question. Like MMdel said, the relief valves are in the reservoir under the fuel tank.

There are two, one for 'up' stroke and one for 'down' stroke. One is in the end of each of the twin spools in the housing. (Those spools are precision fit to their bores. Don't mix them up.)

how it is supposed to work is when the pressure gets too great or the cylinder reaches the end of its stroke, the valve opens and allows some of the oil flow to push the spools back to the neutral position, where oil can flow but there is practically no pressure. This is the only pressure relief the system has.

There is also an orifice in each one that allows the oil to flow back into the reservoir. If you try to hold the lever, it forces more fluid into the relief valve, and the oil really squeals when it goes through the orifice.

MMDEL · Apr 15, 2011

Sounds like a plan. Good Luck!
MMDEL

karl f · Apr 15, 2011

thanks for the info, t.r.k.

karl f

karl f · Apr 22, 2011

The leak appeared to be installer error/component
failure. The rubber ring looked pinched on a spot
of the inside diameter and the thickness was uneven
throughout. Repaired with modern o-ring that fit OD
of flange bore.

karl f

U hydraulic relief valve

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