Here is a video (some of you have probably already seen this) of my old quincy air compressor. The unloader valve cycles but it can do this for a few minutes before it shuts down. It does NOT build up too much pressure. The diaphragm of the unloader looks fine, old but not cracked and seems like that part is actually working. Im starting to think it is the pressure switch but not sure and dont want to tear into it before i have a better idea of how or what might be the actual trouble.
https://youtu.be/xiPihdiys_g
Jim