Stop in for a cup of coffee

We can’t place an exact cause of the slow returning break pedal. It hangs up for a second or 2 before releasing. The car is not pulling left or right. The breaks are not hot. You can feel the breaks stay engaged when hitting the gas before the pedal releases but all of them stay engaged. There are disk breaks on the front and drums on the rear. The master cylinder that is in the car is a used master cylinder that the previous owner had pulled from a junkyard from a newer vehicle. I am wondering if there is something else to check or do I just replace the master cylinder.