2007 Silverado Classic. Brakes have always been sort of suspect on this truck. A few years back I had to replace the brake booster and master cylinder. The master cylinder was leaking into the booster. Not immediately after but not long after the pedal developed a groaning noise when pushing the pedal and releasing. Sounds like when you stretch a balloon almost. I thought maybe the rod going to the master cylinder was rubbing something. I put a small cut in the boot and spayed some wd40 in it. I cannot figure this out and everything I find on the internet for a groaning pedal always ends up being noise from the abs or outside components.