if you have one cylinder stopping and the other keeps lowering i would check the mast real good and make sure the channels are straight, check inside the tracks and make sure there is nothing caught hanging up a roller.

you say one cylinder keeps lowering and the other stays... what happens when you keep holding the lower lever? does the other cylinder eventually start going down? These cylinders are tied together and share the same hydraulic supply source so fluid is balanced between the two to apply equal force between the two for even raising and lowering.
If you do not find anything irregular in the channel and roller area then you may have an issue inside the hanging up cylinder causing it to bind.