2 x press/vacuum relief valve, part #48935K25 from McMaster Carr $7.70 each
1 x Solid gas can lid.
1 x 1/4" drill bit (pilot hole)
1 x 7/16" drill bit
1 x 1/4 NPT Tap
Some form of tap wrench
Mark position for two holes for the relief valves, I found the center of the cap, and then marked the holes 1/2" to either side of the center.
Mount the cap in a vise or clamp to a flat surface.
Drill the locations with the pilot hole drill. (Don’t drill through your flat surface)
Drill the pilot holes out to 7/16"
Carefully tap the two holes from inside the cup side of the cap, making sure to keep the tap as square to the cap as possible.
Do not run the tap all the way through; this is a tapered thread, stop tapping the hole with about 1/8" to 3/16" before the end of the tap.
This will allow the relief valve to become tight before bottoming out the threads.
Apply some Teflon tape to the threads on the relief valves.
Screw in the two relief valves into the cap until they are tight.
Dis-assemble the relief valves one at a time, to make a pressure valve put the ball under the spring. Re-assemble the valve and screw down the adjustment screw, blow into the valve while screwing the valve in until you can no longer lift the ball with your breath. Tighten the lock nut.
To make a vacuum valve, put the ball on top of the spring. Re-assemble the valve and screw down the adjustment screw, Suck on the valve while screwing the valve in until you can no longer feel air passing through the valve. Tighten the lock nut.
If you have a controlled pressure and vacuum device you can use that to set the valves.
That’s all there is to it.