Brake pedal vibration

Front wheel bearings? Are your front tires cupping?

 

Disc brakes? Or drum?

 

if drum brakes, could be an out of round brake drum (or drums)

if a disc brake, could be a warped rotor.

 

Maybe hydraulic brakes you would get back feed into brake pedal for out of round drum. Would be the first I ever heard of in an air brake system. But hey, who am I lol

 

At least one of your drums are bad. There isnt anything else i can think of capable of transferring pressure back to the service brake pedal. However if that is the case the whole truck will not be feeling well during the braking.

 

If I thought about this (You welcome..) a bit more you probably will find that at least one drum has crystals on the lining looks like giant cracks inside the surface is what you want to look for. Those drums are automatically needing replacement without question. If you find them only on the trailer and never the tractor that tells you that whoever is hauling the trailer is abusing the trolley to save his or her tractor brakes. Usually a owner operator or lease etc. But sometimes company drivers get in the habit of the same.