Does the 3 pt leak down when you turn the tractor off, with a load or without a load?
Sounds more like the 3 pt cylinder is leaking through the seal. The
FEL has its own seals independant of the rear 3 point. They only use the same pump, so if the
FEL is working the pump is working. You are probably in the early stages, as the leak becomes bigger the 3 point will leak down with no load.