Dan -
Here is my experience. Early on I was drove overly defensive to avoid that lock up situation. As time has gone passed I have picked up the feel during heavy breaking when I actually have to let off the brake even though I still have to get stopped. You really sense the air building up in the system.
I will say these things do stop quick if needed, had some chocolate brownies go flying during a emergency stop and thankfully they landed on a carpet running which was easily replaced.
Bill
2003 28' Show Hauler Motorhome on a 1995 FLD 120
www.showhauler.com