Lots of people have some type of on board air, ranging from York and caddy compressors to C02 Power Tanks.  It's a great idea for airing up tires in the middle of the woods, re-seating beads, and running power tools.  A friend of mine runs an old fire extinguisher as a C02 tank, and I thought this was a great idea.  I followed his route and picked up a used 10 pound fire extinguisher from F and M fire protection in Hyattsville.  Those guys were great and answered all my questions thoroughly.  The tank came inspected (good for 5 years) and filled with a new valve stem.
I also picked up a bracket while I was there for secure mounting in the bed of the truck.  All I had to supply was a regulator and some hose.  I ordered a 120psi constant regulator from William's Balloons out in California.  Then picked up various fittings and chucks from Home Depot and auto stores.  I ordered some hose from Air Hose Supply and a gauge from ...somewhere.

One of several reasons for me going this route was cost.  The tank was much cheaper than the popular Power Tank.  What I don't get is a shiny tank (not important to me.  The truck is scratched and dented, (so who cares about the C02 tank?), no regulator with a gauge (the pressure is constant and I'm not worried about changing the pressure at the hose right now), and no handle (won't really need to move it around much) among other things.

I built this system for about half of what a Power Tank costs.  I'm not bragging, if you have the $$ for a Power Tank, hats off to ya.  I'm on a budget though.  Robert's Oxygen wanted $106 for a 10 pound tank and $20 to fill it.  No thanks.  Another welding supply place had a 10 pound tank for $75.  I could have picked up some caddy compressors at a junkyard, but this is time consuming and I don't get to places like that very much.  If I do go to a yard, I'd rather look for Toyotas than caddys.  A York compressor was another option, but too expensive and no good places to mount in the engine bay.  Plus I'd have to bolt an air tank somewhere in my truck, which I didn't want to take the time to do.

So now I can air down tires with my hoses and air up when I'm done.  I now have a schrader valve connected to a quick release so I can air up at a gas station if I don't feel like using my C02.

Drawbacks:  I can run out of CO2.  I just have to keep an eye on the weight of the tank and make sure I have enough left.  I'm not worried about this right now, since I don't wheel as much as I like.

Cost breakdown of CO2 setup only:
Refurb tank: $65
Bracket:       $35
Regulator:    $30
Hose (3/8):  $12
Total:          $142.00

Fittings:
male plugs $1.40 x2
quick release coupler $3 x2
360 degree swivel $10
Locking ball foot chuck $5.00
1/4" brass nipples $1 x~3
Total: $26.80

I made a hose setup so I could air up (or down) 2 tires at once with my C02 setup or from a gas station air pump.

Hose setup:
Hoses: 1/4" NPT 12 foot 300psi hose $15.00 x2
Fittings: 1/4" NPT male plugs  $1.40
1/4" NPT coupler (quick release)  $3.00
Inline shutoff valve $3.00
Brass hex nipples $1 x~3
4 way female 1/4" connector $3.00
Schrader valve fitting $2.70
Locking ball foot chuck $5.00
Gauge: $3.50
Teflon tape: ~$2.00
Total: $55.60

Grand total: Yikes! $227.40, give or take about $5 - $10, because I probably missed or over counted a few small fittings here and there.  For a stripped down version, you can skip the special hose setup I made, and cut cost there.  One thing I found was that Air Hose Supply runs sales every once in a while, so you might find some deals on air hoses and fittings there.

Most of the fittings I bought at home depot, if I couldn't find them there I went to places like Autozone, or online at www.airhosesupply.com.  The 25 foot hose I picked up at Tractor Supply Inc., but you can get those at home depot or Air Hose Supply.  I bought the other hoses online, since they were a not-to-common length.  I keep all the hoses wrapped up in an old Detroit Lions gym bag, which is just a bit too small but still holds everything for the most part.

At the end of the East Coast 4Runner Jambo 2002, I used my setup for the first time in a real situation.  I borrowed a wrench from Allen D. and tightened down my regulator, then aired up 2 tires at a time.  What a cool setup!  I did a rough estimate, but I think that it took about 30 seconds for 2 32" tires to go from 22 psi to 35psi.  Not bad.
The air hoses worked great, there was more then enough room to work, and my 360 degree swivel came in handy.  The regulator never got frosty (but the swivel did, no problem though).
This a slick setup, I must say.  If you are worried about releasing CO2 into the atmosphere, I wouldn't worry about it too much.  There  are plenty of other major sources of CO2, so this is less than a drop in the bucket.  (and I'm sort of an environmentalist...)

Below are some pictures of the setup:

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