The installation you describe sounds more or less the same as that in my B-55 Baron. I had boots installed about 6 years ago, and had similar problems getting the instrument air pressure set properly. As I understand it, essentially if a Baron has boots, the primary air system is a high pressure system that is set up to operate the boots. The instrument air is provided by a take off that is in essence a pressure reducing valve located behind the lower sub panel of the interior on the pilot's side with low pressure plumbing to the instruments.
In my experience the problem you describe is likely the cause of that pressure reducing valve failing to function, which I would suspect is caused by:
- A defective valve;
- Excess pressure on the high pressure side interfering with the valve;
- Contamination in the high pressure system with particulates interfering with the pressure reduction valve.
I would have my shop replace the valve on a warranty basis (failing after 15 hours is ridiculous), check that all the high and low pressure settings for the boots and instrument air systems are properly set (i.e. at the pumps, the valves, etc.), and thoroughly flush the system (high and low pressure) to eliminate contamination.
The people at BF Goodrich in Akron, Ohio know this system inside and out.