[77newell]

I would think there have to be a couple leaks, one in each of your brake systems (front brakes, rear brakes) for the symptoms you describe to occur. Air comes from the compressor into the "supply" tank, also sometimes known as the "wet" tank. Air then goes through separate lines and check valves into the two brake tanks. The rationale for this arrangement is that a problem in one brake tank should not effect the other tank. However, a leak in either tank will also drain the "supply" tank. The only place where the two brake systems are involved with a single component is at the foot valve, otherwise they are completely separate. With the brake pedal released you should detect no air venting from the back side vent on the foot valve. If there is any other internal problem with the foot valve it would gradually apply brakes and you would have noticed that.

The source most likely to cause leaks with the fewest components would be the tank drain valves. If they are the one automatic ones, I've had problems with those. Not bad problems, just annoying ones. Check if air is venting from these drains.