I didn't look at that drain today to verify whether there's a hydrostatic valve there. But I'm pretty sure there is, not least because when I drained the pool completely last month water would seep back into the pool via the MD, and very quickly. The water table is quite high here. I did try to remove the MD grate when the pool was empty, but there was no obvious way to do so.

Bottom line at this point is that it appears that the first order of business is to control for this valve. Since I don't have scuba gear, it looks like I need to drain the pool again, remove the BD grate, and just go ahead and replace the valve using the valve tool they sell at that site you linked to above. I guess I'll buy a new grate, too, since I might have to destroy the one which is on there now to get it off.

But my inability to get the pump to keep its prime if I try to pull more than about 25% of the water from the bottom drain concerns me. It seems like the only circumstance which could account for this would be a hole in that MD pipe.

A somewhat related question. I noticed a new problem yesterday on my equipment pad. When I run the pump with the selector which is on top of the sand filter set to either "Filter" or "Recirculate" some water leaks out of my exhaust drain. It seems like the handle is pretty hard to turn on that selector. What might be causing this? This just started yesterday. Here's a pic of the flow with the selector set to "Filter" : https://drive.google.com/file/d/0B4q...it?usp=sharing