Manual Cab Pump?

What am I missing? I'm on my 3rd manual cab pump. All of them leak out of the black plastic housing on the back within 10mins of installing them. All 3 have been "new" pumps from one of the "big box" vendors.

plastic reservoir? Havnt seen one with a plastic reservoir. The oem lmtv pump Is all metal. Because The return fluid is forced there to keep that reservoir full, before returning to the air op pump, the hand pump reservoir must be sealed and able to withstand a little bit of pressure as the return fluid flows…. It could de that you have a steering check valve issue that is causing excessive hand pump reservoir pressure…

@roverchef is correct. The new manual hand pump has a plastic reservoir. I purchased a NOS hand pump to replace mine (which turned out to not be bad) and it has a plastic reservoir, which I found odd. The original pump was all metal.
My issue with the hand pump was with the manifold. I removed and cleaned the filters and removed all the hydraulic valves and cleaned the small passage ways behind the valves.

Well there are a pair of steering checks with 2 different psi cracking pressures(1 & 3psi I think) that force return fluid to prefer the hand pump reservoir until the pressure builds enough to exceed the crack pressure of the second check.

if that check was clogged it could cause excessive pressure on the reservoir…

Here is a pic, it is a 5PSI check valve(circled in red) this forces fluid to the hand pump until it builds 5PSI to then allow the fluid to return to the air op pump reservoir. The 1 PSI check allows the hand pump to draw fluid from the AOP reservoir if it pulls into enough vacuum to open that check…

You could remove the fill plug on the hand pump and put in a low pressure gauge with a rubber plug and then lower the cab to see how much pressure is being applied to the hand pump reservoir by the returning fluid… You Shouldn’t see more than about 5 PSI… That check is the only thing I see in a properly plumbed system that might cause excessive hand pump reservoir pressure. A damaged or kinked return hose to the AOP might also cause it…

Thanks y'all. I post my findings.