[Andrew Bates]

To answer your main question, The only legal configuration is the one that meets all of the requirements set out in the game manual.

Specifically the rules governing pneumatic are in section 4.1.10.

I feel you are asking about whether you can use plastic fittings or brass fittings. The answer is for the most part you can use either. Rule R86 requires that you couple the relief valve directly to the compressor. For this connection you would use brass fittings. The wording of the rule and it's enforcement last year indicates they don't want it connected via flexible tubing.

The only other place you would likely use brass fittings is to connect to the high pressure gauge, pressure switch, and dump valve. Teams with typically assemble these three items together into one assembly with brass fittings. However you could conceivably leave them separate and connect them together with tubes.

Good luck.