silverado brake pedal goes solid, please help!
 

i have a 99 silverado 4.8 liter. the problems is that every once in awhile fortunately at slower speeds the brake pedal goes solid. i think more solid then ifit was just loss of power braking. you can almost stand on it and noting. you have to let off and then reapply the brakes to get it to stop. it seems to do it mostly at slower speeds, like pulling into parking lots or up to stop signs. exactly where you dont want it to happen. i know the truck needs and EVAP valve? but didnt think this would have anything to dom with the brakes. any help is greatly appreciated as my girlfriend drives the truck quite often as well.

thanks

Brake booster could cause that.