Drain Fresh Water Tank?

This is odd.  I don't have a schematic of the fresh water tank but what's happening doesn't make sense to me.
Pumping air (below 40 psi).
Water does not come out of tank drain (low point) but "spits" out of overflow hose (high point).
If drain valve is opened, water ceases to come out of overflow and a small amount comes out of drain and stops.
Drain valve closed and goes back to overflow.
Has anybody seen this?
(It's going to take forever to empty the tank this way.  Compressor may get too hot and quit!)
Thanks,
Peter