Here's what you should start with:
Two 1/2" CPVC endcaps, a small section of 1/2" cpvc (enough for the two endcaps to sit as if connected directly), some super glue, a brass stud (found in the area of Ace where they have the drawers of screws and nuts), a 1/4" to 1/2" rubber stopper (found in the same area), and a small spring that will fit underneath the stud's head. Not pictured but necessary are two lengths of 1/4" OD vinyl tubing, goop, and a drill.
That's the stud as it's sitting with the spring on it. I used a part of an AT3K trigger spring, which is suitable but for the next iterations I plan on using something a bit stronger, or the same trigger spring doubled-up. The goal is to have something strong enough to help the seal but weak enough to be easily depressed, somewhat like the SMDTG trigger.
Drill a hole in one of the endcaps wide enough to give the brass stud some wiggle room. Your goal here is to allow the stud to sit properly while giving the air enough room to escape the chamber, which is the point of a backpressure valve.
Take the rubber stopper and drill a small hole about an eighth of an inch deep on either side. I would recommend the wider side but it's not necessary (in supergluing the stud in place, I put too much glue in and had to cut off part of the washer; I resorted to using the smaller side for this trigger which hasn't changed the mechanics or seal of it in any way). You want to give the brass stud enough room to sit while having a satisfying amount of trigger movement.
It should sit like that.
Take your other endcap and drill a hole in the bottom, just wide enough to shove some 1/4" OD vinyl tubing into. Super glue it in place and goop it once the super glue has dried.
Not pictured here (but seen in the final product): You'll want to put another hole in the side of the endcap, not too far from the bottom, in order to put a piece of tubing in for the output. This will save you a vinyl tee, which is nice.
Here are the final pieces before assembly. To glue the stopper to the stud, take your super glue and put a VERY SMALL DAB at the very bottom of the hole you drilled. You absolutely do NOT want ANY SUPER GLUE on the top of the stopper; it will stick to the top of the endcap and ruin the seal by solidifying the rubber. This is not at all ideal in any way and you can not let it happen. If it does, cut it off and try with the other side of the stopper.
Use some adhesive (preferably CPVC cement) to stick the two halves of the trigger to the single piece of CPVC you have. The trigger should be able to take as much pressure as you're putting on your tank. Let it sit.
And voila!
Beautiful. I tested mine with a Titan pump immediately after sticking it together and it unsurprisingly blew apart. I had to reglue it but it does mean that it sealed perfectly. Enjoy.
Edited by Ice Nine, 18 February 2010 - 01:15 PM.
