1999 Chevrolet Silverado 1500 "99 suburban bleed brakes" Questions

99 suburban Brakes bleed out fine when the car is off. As soon as the car is started the brake pedal goes to the floor and won't firm up??? Sounds like your brake master cylinder has gone out.  

...99 suburban, but replaced the front pads on my 99 suburban, but pedal fades when step on brakes. when truck is turned off the pedal will get hard. Thanks - (need to fix it so can take kids to school ...