Do the rails continue lowering while the forks remain stationary? If so, the mast isn't staging properly. Innerslide lube and repeated lifting-lowering usually fixes this (in older, seldom used trucks).

If the forks continue lowering just as if the switch was still engaged, then you might have a sticking solenoid. On some old solenoids you could flip the coil and counteract the problem of a magnetized valve body. You could try swapping out the solenoid if you have a sister truck.