Rubber tip on the brake rod
Fuel line will get soft and the rod will come out eventually and you will not have brakes, To install that rod you must remove it from the pedal under the dash. Install it into the master cylinder and pop it in place with force. You will not be able to get it out easy and save that ring. So if you have a boot and plastic cover put it on the rod first, usually if the rod came out that easy it had hose on it instead of the proper material ring. .