In the past year, the calipers, master cylinder, brake booster have been replaced. Recently as two weeks ago, the pads, rotors, on the front and a couple leaks in the rear brake line have been fixed as well as the abs sensor have been replaced. In the past few days I have noticed only on the highway, the pedal would be hard as a rock and the truck would pull left but at slower speeds the brakes would be normal. Now in the past 2-3 days, the hard pedal, pulling to left, and humming noise has increased. Still under 25 mph the brakes are normal. Can someone please give me an idea of what I am dealing with? thanks Zack.

 

The hard brake pedal sounds like a vacuum problem. The high rpm on the engine the lower the vacuum at the manifold. Not sure how that would equate to a humming problem, though.

 

Is this Humming noise more like a hissing noise? if so sound like the brake booster is leaking I just had same issue with my 96 only time I heard the hissing was when stepping on brake pedal.

 

Not brake booster I have already went through that and replaced the master cylinder and brake booster at same time and bled master cylinder as well. It is a hard pedal that goes back to normal at lower speeds. The humming is more like a cheap air motor running.

 

The brake booster depends on vacuum to give one that easy pedal feel.

Again, at higher rpm's on the engine there is lower vacuum at the manifold. At lower rpm's there is a higher vacuum. From what you have stated, you still have a vacuum problem, not a brake booster problem.

 

I don't even know where to begin, I know a large vacuum line goes from the intake manifold to the brake booster. Where else can I look for issues? How can I test or troubleshoot the vacuum system?