1970’s Model 880 help

I posted this question under "Ted Williams help".  I figured out it is the same rifle as an 880 model.  I'm hoping to attract some more help.

My problem is with the chamber valve seal or pump abutment.  When pumped, it will hold and fire one pumps worth of air.  However when the pump lever is moved back for a second pump, it lets the air back out.  There is no o-ring or gasket, it is a metal piston that seats or a metal ridge in the brass air chamber.  I realize they can be bought for $50, but I enjoy the older models.  I can't think of anything else to try short of buying a new one for parts.  Any help would be appreciated.  Thanks.