Without any warning of any type, while driving at 65 miles per hour on a U.S. interstate, the vacuum pump completely died, leaving me with very little braking ability. The brakes became very hard to press, making stopping very slow. If someone with less leg strength were driving, such as my daughter or wife, they'd have struggled or maybe have been unable to stop, crashing and maybe dying. GMC is aware of this issue but has not stated why these pumps are dying without warning and in the middle of driving. There is a bulletin, so your mechanic should be able to figure out what may be happening quickly.