We have been having problems with our pressure regulators leaking air anybody know why this is happening? 
try tightening the bottom connection where the pneumatic tubing would go out to the solenoid
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try turning the bottom portion to the right to tighten it. This happened to my team.
Hope this helps!
as well, you may want to press all the tubing in tightly. my team didn’t the first time and our robot hissed all the time when we drove it.
twist the bottom of the regulator ajo518 said. if that doesn’t help i would check the seal on the threads. on pneumatics there is a sealant put on it (from what ive seen usually white) which helps stop air from leaking through the threads.
hope that helps
If none of these work cut a short piece off of the end of the tubing ensuring that the end is square. This sometimes helps the seal.