I use "Push to Connect" fittings from McMaster-Carr. Some call them "Poke-n-Hope" fittings. There are less expensive mfg's of these fittings on places like amazon, but those that I've tried all leaked. If slow leaks would bother you buy the air brake version of the fittings as they have a sleeve to fit inside the tubing and keep it from compressing away from the seal.
I've used both polyurethane and nylon tubing, can't say one has any advantage over the other. The nylon *might* be less inclined to compress when unpressurized and then leak for a bit under pressure. Again, the air brake version of the fittings are what you want.
This is the type of fitting supplied by ARB, only ARB uses 5mm sized tubing. Which nearly DNE in the US except for thru ARB. 4mm is extremely close to 5/32", enough so that I've had no problem interchanging tubing and fittings in an industrial setting. (We run a lot of automation on compressed air at my work.) I changed the ARB fittings on the pair of air lockers in my '88 4runner to the 4mm - 5/32" size and have had no problems with the slightly smaller tubing. Can buy those sizes nearly anywhere in the US.
I use NPT bulkhead fittings from Mcmaster to attach the air hose QD's to my vehicles. QD on one side, tube to NPT adapter fitting on the other. The first two of my vehicles to get OBA had a QD at each bumper. In a thread somewhere on OBA installs a guy proposed that instead that the QD's be put in the middle of each side. That makes so much more sense to me that the next install will be done like that.
I use industrial adjustable pressure switches from Automation Direct to control system pressure. These go a little north of $100 each, but they just work and I don't have to fuss with them. The switch on our camper has been in service for nearly ten years and I've never had to do anything with it.
I put it and an over-pressure blow-off valve (from McMaster) and the regulator mentioned below in an aluminum manifold (from McMaster, starting to see the theme here?) somewhere away from the compressor's hot discharge.
Which brings up a point, the hot discharge from just about any compressor can melt most polymers. I use 3/8 copper tube (from the local hardware store) with steel JIC 37° fittings, nuts, and tube sleeves for the hot discharge air. Using the copper tube I build a bit of an "aftercooler" so that by the time the compressed air reaches the storage tank it has cooled down considerably.
On tanks I've lucked out. The first system has parts harvested from an air ride low rider that I found in the junk-yard. The second system uses a small tank that the Puma electric compressor came with. The next system will use an air brake tank off of a big rig. Can buy those in various sizes usually for less that the commercial OBA & air suspension folks sell them for.
On the ARB supply vs the rest of the OBA system, I set the pressure switch to 125-130 psi, and then use an industrial pressure regulator (from McMaster) to set the ARB part of the system to 85-90 psi.
I won't claim that mine is the best OBA install, but I dislike having to work on systems soon after they are built. I keep looking for better parts and ideas so this is a summation of where I'm at right now.
