Is a PVC union air tight?
PVC is not rated for and not safe for compressed air service, with the limited exceptions of "if buried in the ground or completely screened in wire mesh to catch the shards." 

It makes lovely shrapnel - it has injured people. Nice and cheap until it costs you something more dear than money. 

Here's a 480 PSI (water, not air - no PVC is made or rated for air service) that worked for 15 years and expired at 175 PSI (air) or less.


Link for the above picture. The writer of the link may have "learned" the wrong lesson - even at lower pressures, the pipe degrades and fatigues in air service, and fails in a fashion that is dangerous.

An OSHA link from 1988.

A pipe manufacturer page

Some more reading/experiences/etc...