I hav an 01 2500. the lift pump went out on this truck and it sat for a while. I put new pump on it, and now that its back on the road the brakes are not working right. Sometimes they work fine, some times it feels like the abs kicks on and the pedal is rock hard and will not stop me. Could the vacum pump be the problem, or could it be something else. just wanting to see if any one else has ran into this.

 

Well assuming the brakes were fine before. You may have a vacuum leak at the brake/power booster and when you loose vacuum you have the good old manual brakes.