Re: bumper as an air tank
I've built several of them - but I always suggest not carrying them with pressure in them. In an accident they could rupture and become a bomb with shrapnel.
But - when you stop to air up, fill the bumper/tank from the on-board air while you are getting the hose out. A bumper isn't enough to air up 4 tires, but during that "dead" time, and the "dead" time moving between tires makes it go a little quicker - or seems to.
But - usually an on-board air supply alone isn't quite enough volume to run a good impact - the bumper tank helps considerably.
Just bleed it off when you are finished.
In fact, I'm probably building a few more soon.
You can't beat the unlimited supply of air from an on-board air system. Once you have it you'll understand.
You can spend lots of $$$ on an on board air system, or build one for cheap. I've found the Sanden air conditioning compressors do great - that's the round one used on later model Jeeps and lots of other cars. You can get them for about $20 at most junk yards (they seem to think the Yorks are made of gold!)
They are small enough to usually fit, the hardest part is setting up a belt to drive it. Sure, the York is faster, but only by a few seconds. Usually 8 CFM vs. 10 on a York.
I saw an even smaller compressor on a small foreign car that I was intrigued with, but didn't get one. It may work out even better where space is limited.
Compressor $25?, switch $1, check valve $10, gauge $5, and a quick coupler $2 is all you need. Everything else is luxury.