Re: drive slowly drifts down...
plugging the ports on the rams is not a valid way to test a double acting unblalanced cylinder for internal leakage. Even if the piston seals are totally gone, the rod will only retract a little bit- because you are adding rod as it "drifts", and oil doesn't compress, it wont retract all the way. The oil leaking past the seal from the base end has more volume so it doesn't fit in the rod end...eventually the whole cylinder just pressurizes and holds the load.
This in and of itself rules out leaking piston seals ...*sort of*... combined with a slight leak of the thermal expansion valve or relief valve, a bypassing piston seal can sort of work together with the leaky valve to make the cylinder drift in.
There is also a "manual release" valve on Cobra trim pumps- the manual release valve is there to let a technician lower the drive without d.c. power and without spilling oil - it basically opens a leak from the base end ("up") port to the reservoir so the drive will drop. make sure that hasn't been backed out - I think it just takes a slot screwdriver and is sort of beside the one motor housing mount screw.
Besides, trying to purge every bit of air out while installing the plugs is impossible, and air does compress, so you'll just end up more confused.
The easy way to check for one bypassing ram is to simply listen with a screwdriver handle to your ear or a stethoscope if you have one while someone raises the drive or attempts to while it's already fully raised. You will hear if oil is bypassing the seal on the piston.
I been making my living working on hydraulics for 20 years and I now teach it for a living.... your ears are good for diagnostics...
I get the students to remove the seals from a cylinder and then we try to make it retract...
it won't - oil only compresses .5% at 1000 psi.
A balanced (double end rod) cylinder will retract, but sterndrive trim cylinders are unbalanced (more volume in the base end than rod end)
In case it is ever suggested, what you DON'T want to EVER do is plug the rod end of the cylinder and try to use the pump to raise or extend the cylinder by using the pump to force oil in the base end (large effective area end). This creates a pressure intensifier and can blow the cylinder open dangerously.
Be safe when working around fluid power. Safety Glasses are a must, goggles better - never go near a pinhole leak (indicated by misting of fluid around high pressure connections and hoses) High pressure mineral oil can penetrate your skin and get in your bloodstream, causing all kinds of problems. One study quoted at FPSI reports 48% amputation rate with skin penetration accidents with fluid power.
