I am pretty sure there is a check valve. Some don't have them but even the old Champions I've seen do have them. In your case if the check valve was leaking it would likely be leaking out the front centrifugal unloader. I have seen that on some Champions. You would hear this when it shutoff but since the tank holds pressure I didn't raise that problem.
Die grinders usually require true 5-7.5 HP horse for continuous use. If you can get enough tank to suit your needs, that is another option, and just let the compressor catch up when you are done. I have 10 HP / 240 gallon and that is a happy medium for me. I could use more air as my angle grinders keep the compressors running non-stop when grinding, but at the same time I just put on ear protection and rather than let them cycle on/off all the time they just run, which is better for the motors. Single phase motors have more moving/wear parts than 3 phase....