Hey GringoPhil: sorry to hear of your troubles. Either the compressor isn't putting out enough air or you've got a large leak. If once pressure was previously built up - though slowly- it didn't drop precipitously once the compressor unloaded (turned off) then you don't have a leak large enough to cause your symptoms. Another way to test this is to hook up shop air to the system, charge it up, shut the supply off and see how fast the pressure drops.
If you find that a leak is not the problem then I would be exploring the compressor and its governor. Bendix has provided a great troubleshooting guide for this in the following PDF file:
http://www.bendix.com/media/document...otingguide.pdf
This guide has some relatively simple test procedures for determining that the compressor is or isn't working OK. If it is OK then the governor is gone to .... wherever it is that governors go when they are shot.