Motorcycle Repair: shovelhead brake problem, hydrolic, master cylinder

Question
QUESTION: Hello.  I have a 1970 Harley Electra GLide with rear brake problems.    Sometimes it works, sometimes not.    (Not a good thing for brakes.) I have replaced the master cylinder and the brake still works intermitently.  There is something on the left side of the rear wheel that has to do with how this functions, but I can't recall what it's called.  Can you tell me what it's called and how to repair it?

ANSWER: I want to make sure, do you have hydrolic or drum barkes?
Mike

---------- FOLLOW-UP ----------

QUESTION: it has a hyraulic rear brake.

Answer
You must be refering to the brake caliber on the left side of the wheel. If you dont have any leaks and you have replaced the master cylinder, then you probably have air in the brake line. You need to bleed the brakes for a while, since the hole in the master cylinder is really small and it takes a while to get all the air out.
Good luck and happy riding
Mike