The first remedy is to replace the o rings. If it still does it, then the A Valve. If it still does it, the pump base needs to be replaced. I sure hope the o rings fix it!
To remove the A valve since that part of the base is in the way you need a 7/8" or 1 1/8" deep socket.
Check the O-ring at the base of the pump housing. If it is cut or broken, it would cause 'leaking down' of the blade...while you're in there, also check the packing cup to make sure it is in good shape.