Own a 300sd 1979(w116) and yesterday I noticed something weird. I started my car and while it was warming up I left my foot on the brake pedal for about 1 minute. As I began to shift from Park to Reverse the brake pedal went straight to the floor. Shocked, I immediately pumped the brake 2 quick times and the brakes grabbed on again. I then waited a few seconds, without my foot on the brake pedal, and proceeded to back up without a problem. I drove 7 miles home without any brake problems and then tried to replicate the same steps at home but without turning the car off. I think I might want to repeat all the steps and shut the car off and then start it again. Just wondering if anyone has ever experienced this situation. I replaced the brake pads and rotors 2 months ago and was driving the car twice a week before the incident. Any advice on what I should be checking for? Do I have a vacuum leak, master cylinder problem or brake booster issue? Im going to bleed the brakes to see if I have any left over air in the line.

Thanks