I know this is an old post, but I have a '91 GTP with the same problem. The powermaster III system uses the ABS pump to produce the power assist for the brakes. The assist is hydraulic rather than vacuum. My ABS pump runs for more than 40 seconds before shutting off when I start the car for the first time in the morning, and it takes a while for the red brake light to go out. With my foot on the brake (applying moderate pressure) the pump runs too frequently. It runs for about 6 or 8 seconds before shutting off for about 20 seconds. The cause of this problem is a bad accumulator, which is the canister on the master cylinder with a yellow caution message warning the technician about gas under high pressure, and has a hex head on the end of it. This system was used between 89 and 91. I am currently looking for an accumulator. GM parts direct has one for around $150. A little more pricey than I had planned on. I am also going to replace the pressure switch as well while I'm there. Hope this helps.
Andy
