I'm upgrading from a Craftsman 30 Gal oilless to a 5 Hp 60 Gal upright. I was going to try to sell the Craftsman in the local paper, but figured since it still works, I could just plum it together with the new one and have even more air.

I'm concerned about having both compressors tied to the same air line though as I don't want the smaller one fighting the larger one, or does that not matter? If it does, I was thinking about using a check valve, but haven't been able to find any for air line.

I also considered having just an isolation valve, allow the Craftsman to run long enough to charge the tank with the valve closed, and then turn it off with the valve open and use it as a reservoir running just the 60 Gal compressor.

I also have a small 13 gallon compressor with a bad motor. I was going to plumb that one in as well just to have the extra reservoir volume.

Any ideas on a good and safe way to plumb these together?

I've already read through all the threads regarding air line construction, ie, black pipe or copper (NOT PVC!), and routing issues such as up to the ceiling and back down with a trap/drain and filter/oiler, etc, etc.

The question is more along the lines of if I need check valves, etc to control air flow amongst the two running compressors.