I agree the previous posters.
The pump mag works by not allowing the of the bolt to fully reset without manual assistance which is done by the pump. This leaves it slightly forward of its normal position. If the powertube oring can't come far enough forward to press against the back of the bolt stem, residual air left in the chamber will leak out. Install a shorter powertube spacer to allow the powertube oring to move farther forward to seal against the back of the bolt stem.
Except for the Automag in front, its usually the man behind the equipment that counts.