Sorry Wrench3... I agree with you on most things, but not this one....
Let's start with the first part. If a seal/'O' ring is leaking allowing pressurised oil across the piston, then that pressurised oil would be enough to open the UP side valve. Thus it would not stop the drive from lowering. Even if it didn't, the pressurised UP side on the other ram would be enough to open the valve. (Remember, the lines are in pairs split at the manifold under the gimbal housing.)
Second part. If the leak was big enough for the oil to flow from the open UP port without the ram moving when the DOWN switch was pressed, it would drop on its own, without any input from the pump. Most internally leaking seals/'O' rings leak a very small amount, and the only sign that you have a leak is that the drive SLOWLY drops down by itself.
A drive that will not move under pump power is usually because of some external force (like a jammed ram or a locked piston)...
Chris........