Last night I went to the track and notice that car only makes 10psi of boost. I'm using Tial44 gate. When I did boost leak test I have air coming out from the dump tube, when w/g is by passed(no vacuum hoses) it holds boost. When I hook up vacuum hose to the lower port and apply pressure(boost leak test), air comes out of the dump tube. I have disassembled the w/g and found the diaphragm to be OK but the air seems like its coming out through tha piston?? there was some scoring on the piston. what would cause that??. please help..

What spring are you using?

I have the small and large blue spring 20psi

How about the piston itself actually sealing against the body? You said the air bypasses around the piston. How does the contact area of the piston and the body look (aka the mating surfaces)? Any scores there?

Where exactly is the scoring on the piston? How bad is it? Something you can remove with some really fine steel wool or something a little worse? A picture or two?