See attached photos of lift left overnight.

We are going to leave the lift again overnight with a load in the car and mark a point on each cylinder stage to identify which one, if any, of the stages are leaking.

We will also disconnect the fittings before the ball valve to see if oil is bypassing and leaking through the seals of the ball valve.

All in all, I doubt the system could hold about 100mm of air in the system, the odd few bubbles that could cause bounce maybe, but not that much of drop overnight would be caused by air in my opinion.

Very good approaches to further narrow down the error.
The volume change of air during cooling should not be underestimated.

Also check the fill level of the leaky oil canister, if it has one, but this is difficult with a leaky oil pump.

That was my first thought, temperature change of the oil changes the volume. When im evaluating an inground to determine if its leaking underground I take the temperature of the jack into account. Ideally leave it off overnight and let the oil cool and set everything up next morning then come back the following morning to check.

Also if your trying to determine the amount of oil lost from the packing get a marked measuring bucket from the paint area at your home goods store to use as a pit can. There's also quick volume calculators online to convert that volume into what you should see in vertical movement of the piston.

We have tested this when the museum is closed so the lift hasn't been used and the oil is already completely cool.

The lift doesn't have a leaky oil canister and doesn't have a scavenger line hooked up to the barb fitting on the bottom casing either.

When we conduct these tests we are doing it over the few days that the museum is closed.

If it is the ram seals, it could only be from over use. We fitted new seals in 2018 and the museum was closed in 2019 and 2020 due to covid so if it is the seals, they have only lasted 4 years in theory.

The duty cycle Bucher have recently told us that this specific lift is good for with the oil cooling system is 80 runs per hour and we have checked the run count last summer and it was on average 120 runs per hour without a break.

It would surely be deteriorating the seals from this kind of use. When the lifts were installed and the museum was opened 13 plus years ago they had under specd the lifts for the amount of use they would endure and then in 2018 the seals were replaced and then after covid the amount of use has only gone up more each year.

The only other thing we haven't checked when the ball valve is closed is the rupture valve for leaking because after the ball valve it's only the cylinder stages, rupture valve and ball valve in play.

In the photo it looks like there is a hose or pipe connected? Is the Leakage oil hose incorrectly connected and thus the lower stage was not vented?

A mistake during initial assembly ?

I can 100% tell you that there isn't a hose or pipe connected to any of the two fittings. The bottom smaller fitting is an air bleed screw and the top fitting is a barb fitting for a hose but neither have a hose.

So today we sent the lift to the top floor again, shut the gate valve, turned the power off, marked a line at each stage of the telescopic cylinder for this lift and also disconnected the gate valve inlet side so we completely removed the Blain L10 door valve and bucher valve from the circuit.

All that remains is the gate valve, rupture valve, hose fittings and cylinder stages.

After a period of 4 hours we noticed there was no leakage coming from the gate valve but we will still leave it disconnected overnight. We did notice that each stage of the cylinder had dropped around 12 to 15mm in this period and if step into the lift while in this state, it sinks even more and doesn't hold.

See attached photos of what we did.

https://1drv.ms/i/s!Auww3LXVzmjw91waYAv4fxUOF5dW
https://1drv.ms/i/s!Auww3LXVzmjw91ty8MxjA4nHwlg0
https://1drv.ms/i/s!Auww3LXVzmjw92G4SwRojTd-6ubs
https://1drv.ms/i/s!Auww3LXVzmjw92J6RNrQNyY5ZT3h
https://1drv.ms/i/s!Auww3LXVzmjw90s6Xy-DNyFjt_z5
https://1drv.ms/i/s!Auww3LXVzmjw92N3PVZ4vcogaoFX
https://1drv.ms/i/s!Auww3LXVzmjw90zm9Qoos6Y3zh3q
https://1drv.ms/i/s!Auww3LXVzmjw91maWLyf9RIpWuXB
https://1drv.ms/i/s!Auww3LXVzmjw91Tb81v9QIa7zfFL

There was also evidence of a small amount of oil around the first and second stage of the cylinder which was only cleaned off at the end of the last week. I don't know what anyone else thinks bit I honestly can't think of any other explanation except the rod seals causing all of this.

We even synchronised the cylinders before testing and also bled the entire system including cylinder stage bleed screws, bucher mechanical null point bleed screw and blain L10 bleed screw tube.

I will note that the oil was cool on arrival as the museum is closed today, tomorrow and the next day.