I’ve got a 45’ Eagle towable manlift and bottom boom is leaking down.

[oldnslo]

Those appear to be Sun Counterbalance valves. Looking in the nose like picture one that the poppet inside the scallops should pull out a short ways easily. That is the free flow check portion of the valve. If there was a chip holding that open the lift would drift down. In a previous life I could have tested those valves for you but do not have that capability anymore. Probably faint pin stamp on the hex and from memory would start out CBC*-***.

 

[Puppypunter]

Ok so why would there be two identical valves like that? One for up and one for down?how do these valves work? And what would you do before taking it to a shop or buying new valves? And lastly, how would I know which one is actually causing it?

 

[BeezFun]

Ok so why would there be two identical valves like that? One for up and one for down?how do these valves work? And what would you do before taking it to a shop or buying new valves? And lastly, how would I know which one is actually causing it?

I would only expect one valve per cylinder because gravity is the only force involved that can over run the pump. Is it possible things are plumbed so that one of those valves serves the lower cylinder and the other valve serves the upper cylinder? You said there were only 2 hoses connected to the cylinder, I assume one goes up to the cage control valve and the other goes to the base control valve. One thing you can try to figure out what's happening with hose flow is hold your hand on each of those hoses and have a friend operate the controls. You'll feel the hose pulse a little when it gets pressurized, that will help you identify which hose does what.

 

[oldnslo]

Can the lift be put in position where the load would be attempting to extend this cylinder? The valves work in the following manner.
Valve connects to port 2 the lower side port.
Port 1 Nose of valve to cylinder. This port is blocked until a preset pressure is reached or pilot pressure is applied to port 3.
Port 3 Top port is pilot assist and typically connects to opposite end of the cylinder.

When extending oil flows from the valve through port 2 the free flow check and out port 1.

When retracting:
Pilot pressure from the rod end is applied to port 3 this plus the weight of lift and pressure induced load from rod end pilots the valve open and the lift lowers. If pressure drops in the rod end and or load drops below set point of the valve the valve closes and movement stops.

Have you tried brushing the hexes gently to see if there are any markings. They could be faint.

 

[BeezFun]

Can the lift be put in position where the load would be attempting to extend this cylinder? The valves work in the following manner.
Valve connects to port 2 the lower side port.
Port 1 Nose of valve to cylinder. This port is blocked until a preset pressure is reached or pilot pressure is applied to port 3.
Port 3 Top port is pilot assist and typically connects to opposite end of the cylinder.

When extending oil flows from the valve through port 2 the free flow check and out port 1.

When retracting:
Pilot pressure from the rod end is applied to port 3 this plus the weight of lift and pressure induced load from rod end pilots the valve open and the lift lowers. If pressure drops in the rod end and or load drops below set point of the valve the valve closes and movement stops.

Have you tried brushing the hexes gently to see if there are any markings. They could be faint.

Thanks for explaining how those two valves work together. I think these photos are pretty close to the lift that the OP has. The top cylinder is always being forced to retract by gravity loading, the bottom cylinder is always being forced to extend by gravity loading. So aren't both cylinders single acting? I don't understand how the second counterbalance valve would be doing anything assuming what I've said is correct.

 

[oldnslo]

Beez
In looking at those pictures I think that lower cylinder could or would see loading in either direction depending on position of the bucket. A counterbalance valve will prevent the load from running away when set properly. That would explain need for two valves in that cylinder.

One of many times seeing it in person could help answer this question.

Good news is that Sun Hydraulics is still in business unless Ian wiped out Sarasota.. and making those valves. Bad part is exactly which valves are those?

 

[BeezFun]

Beez
In looking at those pictures I think that lower cylinder could or would see loading in either direction depending on position of the bucket.

Yes, now that I stare at it I agree with you. i had to go back and read the original problem description, the only cylinder that drifts down is the lower cylinder, so it sounds like you've identified the problem.

 

[Puppypunter]

Yes when the machine is cradled the lower cylinder is extended, meaning gravity is trying to extend it. here’s what I found (oldnslo I can tell you must have worked in a hydraulic shop or owned one most of your life) because you were exactly right, they r as follows

Sun pilot valve “blue” side

2500 psi

CBCA LAN

9EJ5



Sun pilot valve “red” side

2500 PSI

CBCA LAN

9DG5

 

[oldnslo]

Puppy
The good news is that Sun is still building those valves today. If all the external seals look good I would blow air through from port 2 to port 1. This would flush the check valve which is the load holding portion. Like I stated earlier that little poppet in the nose should move a short distance easily. If it does not that would indicate something is causing it to bind and valve might have to be replaced since tearing them down further usually requires replacing inner O-rings that are a thin metric size.

If load is trying to extend the cylinder the cylinder piston seals could be leaking allowing this drifting. I was incorrectly presuming the cylinder was retracting to lower.

Yes spent 18 years working in various positions at cartridge valve company.

 

[Puppypunter]

Ok so if you would tell me again how this works based off these pictures. on the third photo, that’s showing where the red valve goes in, the blue mark is showing we’re the blue valve lines up. I’m guessing the “R” means retract and the “E” means extend? What does the other one mean? “Bleed?” If you notice on the first photo I’m shining a flashlight into the red valve and u can see light through all three of the ports, shining a light through the blue valve only “R” and “E” shine through. And how does the piping on the side work with all this? I’ll have to go put it back on the lift to tell you whether it is extended for sure, obviously how it’s sitting now is with the boom cradled, but I got to thinking maybe it could be retracted in that position. Thanks again for all your help.