As was mentioned, the volume of air is an issue. All of the 12V compressors I have seen don't move a lot of volume. This works for airing up the suspension because it isn't time critical. But air brakes use a lot of air and can require it frequently. As a cautionary example, in situations where frequent braking is required, even the engine driven compressor sometimes can't keep up and the brakes grow weaker.
As an emergency backup, I suppose it is better than nothing, as long as you don't allow it to make you complacent about repairing problems with the main system. Starting with fully pressurized (120psi) primary/secondary tanks, you will only get a few applications of the service brakes before you drop to 80psi and need to pull over to recharge the air tanks for 10-15 minutes.
12R22.5 tires I've seen run at 95-110psi and take a fair volume of air just to nudge up the pressure 5 pounds. Because of that I'm not sure using a portable tank to add air to bus tires would work. A long air hose would be best. A long power cord would work for a while, but the voltage loss over a long cord would soon burn out the compressor motor.
Most common air tools require consistant high volume and pressure. For example:
- > Campbell Hausfeld 3/4" Impact Wrench - 6.1cfm @ 90psi
> Ingersoll Rand 1" Impact Wrench - 9.5cfm @ 90psi
> Ingersoll Rand 4-1/2" Angle Grinder - 9.0cfm @ 90psi
> Campbell Hausfeld General Purpose Spray Gun - 4.3cfm @ 40psi
> Campbell Hausfeld HVLP Siphon-Feed Spray Gun - 9.0cfm @ 40psi
At a google glance, here are some typical 12-volt, continuous duty compressor's high pressure cfm ratings:
- > ExtremeAire - 1.5cfm @ 100psi
> Thomas TA4101 - 0.95cfm @ 100psi
> Viair 450H - .94cfm @ 100psi