best place to tee into bus air system?

[gus]

My 4104 air system is pretty much original.

The 4104 has no dryer unless one has been added.

It has drains on the wet, dry and aux tanks.

There is no pressure at the ping when the compressor is off and it pulses when the comp is on.

[BG6]

The best place to ADD air is just past the compressor check valve.  Tee in there and you don't have to worry about the valves, distribution, etc.

The best place to TAKE air is from the largest tank in the system. 

If you want to use a lot of air for running tools, put in another tank and a three-way valve between the compressor and the first leg of the distribution -- set the valve one way, and all compressor output goes to the air system, set it the other way and it all goes to the tool tank without draining the air system.