Change in voltage
You need to explain more about just what you are doing, but there is never a reason to use a brake pedal as a ground, because, well,.............it's not.......the brake pedal I mean......It's not grounded, I mean
THINK about it. The brake pedal is a separate piece, with a bushing/ bearing that might/ should have some lube in there, resting up against a stop, pushed there by a spring, connected to the master through a push rod with loose connections on both ends
Did anything I mention suggest a good tight clean solid ground?