1965 Valiant brake woes

There should be a plate with a dust boot that attaches to the back of the master cyclinder. Some masters have a c clip holding the pistons in. You are are probably dont have one since your "original" sutff is missing because of the aftermarket master along with the added booster. That tab shown is more a shipping bracket that holding the pistons inside the master. My original bracket was bent and boot was torn from previous attemps at removing the pushrod. I just made a plate with three holes (both tapped holes and the piston). Also, make sure your push rod has a rubber o ring like gromet which will retain the rod in the master (there is an undercut in the the piston) other wise the rod may fall out if the brake pedal get pulled up accidentally (foot under the pedal when getting in and out of the car, shoe laces caught, etc.).