Brake issues

[Uhater]

2008 gmc sierra 2500hd driving down the road n brake start to apply them self till they are locked up , b4 this the brake pedal has been really hard n super touchy . Had hydroboost tested n they say it passed . Im at a dead end any help would be appreciated.  

[davester]

So all 4 wheels lock up?  And the ABS does or doesn't activate? Have you had any work done to the braking system?