Mustang II brake problem

I am just getting my vehicle out for spring. The brake pedal goes to the floor. So I filled the master and bled all lines and got pedal pressure. I start the vehicle and the pedal goes away. A friend says I need a new booster. He says to suck on vacuum line to see if it holds pressure. It does unless I move the rod that goes into the master....than it leaks air . Is it the booster leaking or ???

What is the correct amount the rod from booster goes into master.? Should it be touching inside master or ???

Rick

I suggest replacing the master cylinder and booster together.