New Master Cylinder?

[firsttimeowner]

Bought my 1981 2 years ago and have had no brake problems until recently. On a drive all of a sudden the brake pedal went to the floor but I could pump the brakes and they returned. This kept repeating and then the brake light went on. Kept driving it and about 3 miles later the brake light went off and the brakes worked fine. Took it to my garage and they checked it out and could find nothing wrong so suggested it might be the master cylinder, but were reluctant to pull it. They suggested I drive it and see what happens. Obviously I am uncomfortable doing that. I am not technical at all but don't see how this could be a one time occurrence. Should I just go ahead and have them pull/replace the master cylinder?