I consider Hector the auxiliary air genius of POG. I will add my 2 cents to this discussion and hope he jumps in here.

Low aux pressure should not result in air dumping on one side (I don't think). If my theory is correct it may be that the loss of aux pressure allowed the Norgren on one side to dump air, but the other side for whatever reason did not shift due to the low pressure. If might be because the Norgen internals are not moving smoothly and as Brian pointed out could be corroded.

I did rebuild my Norgren's on the first bus, but it is soooo much easier just replacing with new. Corrosion internally will mandate a new valve because the rebuilt valve with internal corrosion will never function satisfactorily.