I've seen a lot of posts about people getting confused with how to make one of these things, but here's yet another way of doing it. this particular way dismisses the whole "leaking PVC plastic cap" deal.. I've found this way to be the cheap/best way of constructing a boost leak test. total cost was less than 5 bucks

1- 2.5" or 3" rubber end cap (whatever turbo you have, found at home depot)
1- hose clamp
1- METAL shraeder valve
1- large OD washers, small ID

put everything together as shown. I used the metal valve because it seems to seal the best. You actually screw the two pieces together, versus the rubber one which will leak. Also, the large washer is there so that when you push against the tester when putting air in, it does not create a leak from the rubber giving...

enjoy.