in tank sump help

[dethnode]

I am doing an all in one with an integrated back wall sump behind a black acrylic false wall. In the return chamber, the pump will pump water up and through a bulkhead to return to display. I would like to make water changes a bit simpler however and want to place a "T" on the return plumbing and have the second stem go to a gate valve which connects to a quick connect hose attachment for draining the tank. The problem of course is, that as I am draining the tank, the water level will fall and then the return chamber of the sump will run dry before enough water has been drained out.

My thought was to place another bulkhead lower down on the return chamber that would normally be closed off, but then open it when performing water changes. I would need a valve to connect to this bulkhead that can be opened and closed without requiring much force, as the false acrylic wall will be held with silicone to the glass aquarium, and from my understanding glass and acrylic do not form a very tight bond.

Anyone got any ideas?

[GeoJB]

Any diagrams or pictures? I'm no expert, having a hard time making a mental picture.