what is a cup seal??(I am a true newbie)

[Xander]

a gun on ebay is going for a good price but it needs a new cup seal... wtf is that and how much does it cost???

[Sid]

its a piece of plastic that goes in the valve. The spring pushes against the cup seal closing the valve. The valve pin is pushed in to move the cup seal, opening the valve...not a very expensive part.

[Xander]

where could I buy a new cup seal and sight rail screw??

[Sid]

YEr local proshop.