Hi, I really hope that someone on this forum can help me with this issue as it's really bugging me.

I have an international 354. The lift would jerk (drop an inch and then jump back up) so I took off the complete lift cover and sent it to a local engineering firm that specialise in hydraulics. I asked them to do the seals in the main cylinder and to replace all o-rings in the valve control unit. Well I got it back, and re-assembled it. I fitted a new hydraulic filter and replaced oil with IH hy-tran. However the problem is still there.

I'm aware that the hydraulics on this tractor are the same as the 444, 374, 384, 434 and B414. There is a common test that is recommended for these
tractors to check if the leak is in the hydraulic cylinder. The test is to raise the lift with a weight on it, close the shut off valve, and if it leaks down with
the valve closed then the leak is in the cylinder. Well mine does leak down with the valve closed...so it seems that the cylinder is still leaking...even after the local engineering firm have done the cylinder seals!

Well I'm still really desperate to fix the problem so I've done some more tests. I'm pretty sure that the isolator valve is closing 100%. I replaced the 2 o-rings on this valve. This means that the leak has to be in the main cylinder, right? I'm prepared to buy whatever parts are needed to fix the problem. My tractor has a single groove piston. There is an later 3 groove piston that can be fitted, so I plan to get that one. I'm wondering about the logic of buying a new cylinder also. The reason being is that when I have a load on the lift arms and shut off the valve, it leaks down quickest through the first third of the drop. Then it slows right down. So it seems to me that the the cylinder might be worn in the upper part. Would this be possible?
The last thing I want to say is that there is a thermal relief valve on these pistons. I read on another forum where someone else took this valve apart, installed it backwards and it gave the same jerking problem. It's possible that my relief valve is installed backwards on the piston. But could this cause it to leak down fastest in the upper range of the piston? Can anyone offer any plausible reason why it leaks down fastest in the upper range?

It's such a big job to remove the lift cover and take it all apart that I want to have all possible angles covered before I attempt to do it again. I would hate to tackle it only to find that the problem persists.

Many thanks to anyone who can help.