Master cylinder
 

I have been having issues with my brakes sticking with a hard pedal but will release after a while. I have changed out the calipers, the rubber lines, I have flushed the lines and have good suction. My issue IS I want to change out the master cylinder but they want to know what size bore I needed they have 1.25” or 37 mm which is all little bigger. My truck is an 04 Silverado 1500 with the 5.3 motor disc brakes all around