Anything about hydraulic spool banks on ford 4000 tractor?

When a tipping trailer is connected to the single spool it will tip the trailer up but won't hold it in one position. Is this because the pistons inside the spool bank are leaking or what?

The only way of knowing that is to put a gauge into the quick release and charge the line, do not have any other thing on the line. Do you have two lines going to the trailer? If so the cylinder packing could be leaking and by passing into the return line. If you only have one line then I'd say the cylinder is good and the valve is where the problem is but most times the valve is the last thing to go bad.