@Primussucks thinking about this a little further the behavior you described leads me to believe that filler plug you show a picture of is not vented, if it is pushing fluid out the gap between housing and reservoir. if the pump and reservoir are designed to handle a certain amount of pressure, you could setup a completely closed/non vented system With the cylinder completely full And acting as a storage reservoir. With that filler plug not appearing to be vented perhaps this is what the Manufacturer intended… That filler plug may be a safety relief?
As you pump to raise the cab, the rod end of the cylinder being full pushes fluid back to the reservoir under positive pressure.
As you lower the cab you push a little fluid toward the cab to pull it over the balance point, then the weight of the cab pushing down on the rod and piston forces fluid into the reservoir, the reservoir pressurizes and since the pump valve is lined up to send fluid to the rod end, this pressure in the reservoir pushes fluid thru the pump check valves to the rod end. This would keep the fluid flow for both direction of cylinder operation under positive pressure, but the reservoir must be able to handle some pressure.
the tricky part here is the fill level. Because of displacement, the rod end contains less fluid than the base end of the cylinder. You must allow enough air space in the reservoir to account for this difference. so with the cab pumped all the way up, you would only want a very small amount of fluid in the reservoir, enough to keep the pump suction reliably covered.
the issue here may also be the way the LMTV cab cylinder is configured. it has a safety check in the base end port and a restricted orifice in the rod end port.
the safety check will block flow and lock the cylinder in place to keep the cab from falling if it sees excessive flow like if you blew a hose while raising or lowering the cab.
to keep this from tripping during normal operation, they put a restricted orifice in the control valve that limits return flow to a point below the tripping point of the safety in the cylinder base port. Since you removed the control valve, how are you providing this restriction when lowering?
They also put a restricted orifice in the rod end port on the cylinder to help keep the cab from lurching over the top as it passes the balance point.
If you just connected this PP pump to the cab lift cylinder without a lowering restrictor at the pump,, the restrictor in the rod end port on the cylinder will be restricting flow when lowering, which is probably why the safety isn’t tripping. The problem is that everything upstream of the restrictor when lowering, is seeing probably close to 1500PSI with the cab weight on the cylinder while lowering. If that restrictor is on the rod end port, that makes all the hose plumbing and the pump and reservoir upstream of the rod, so it is seeing all that pressure. The restricted orifice must be on the cylinder base line before the pump. This will keep the pump reservoir from having to deal with that high pressure.
I think this is what is happening with your setup.
i doubt the pump is designed for that much pressure, as some I have seen have a plastic reservoir, and the fluid/air is taking the path of least resistance out the reservoir-pump housing seal…
when I put on my manual pump, I didn’t use a restrictor and it could drop fast enough to trip the safety if I opened the valve enough. I ultimately installed an adjustable restrictor at the pump to control lowering speed.
with a restrictor in the correct location this should work as a sealed system with the pump sealed and only ever seeing the small pressure caused by the volume change due to rod displacement.
This was the first video I did with the prince pump. In it I talk about it getting a little harder to pump as I raise the cab near the balance point. That is the restricted orifice in the cylinder rod end encountering the fluid left in the cylinder from when the cab was last pumped over the balance point while lowering. I didn‘t identify it until after this video was published. This behavior also indicates that the OEM design will not suck fluid to the rod end when lowering as the only fluid in the rod end is that put there during the brief pump to start its downward journey.
the other videos show the restrictor I used instead of throttling it with the hand control…
