Some GM boosters have two diaphragms and a problem with one will cause what you described. That shouldn't happen though just from hard braking unless the vehicle stopped really quickly. There is a valve that falls out of position. You have to worry about that valve if you get a used booster from a salvage yard. Always carry it and haul it in its normally-installed orientation. If you set it on its front or back, that valve can fall out of place and you'll have the same problem. A rebuilt booster will come with a plastic cone inserted to hold that valve in place during transport. The clinker to this problem is the only people who even notice there's a problem are those who have owned the vehicle for a while and know what normal feels like. To other people unfamiliar with the vehicle, the brakes seem fine.
Wednesday, November 11th, 2015 AT 11:00 PM