I have a 07 E6000 lift and the Primary cylinder falls on its own over time and as it falls the secondary system extends. after a period of time the primary lift cylinder doesn't engage until the secondary is lifted to the top then the primary starts to work again for a little bit. then you have to repeat the process of raising all the way to the top and back down again. works this way every time. I actually have 2 lifts doing the same thing the second one started it today.

I am not sure if it is a check valve or a control valve kind of leaning towards a check valve but not sure if its located in the bottom of the cylinder were the line connects. I have not worked of forklifts in about 2 years so sorry if this seem kind of dumb for a question.