When cars sit for a while, things stick. First, push on brake pedal (car running), does the pedal go down, then stop moving or continue to head to the floor. If it stops, the MC may be OK, heading to the floor suggests a failing/failed MC. These units are generally very reliable, but that is with an occasional fluid flush, 15 years of black brake fluid may be too much.

I would see if the brake pads are stuck. Start with the front since it does most of stopping, lift car onto stands and simply take pads out, clean up edges so that they can move in the calipers when you depress the brake pedal, reinstall, pump pedal a few times to set pads, and see what happens (on a safe street). This will happen to my 940 if it sits a while.