1966 pa30. my left brake master cylinder is leaking. need service instructions.

This is generically how to do it. Some aircraft have slightly different hardware. Remove the leaking brake cylinder from the aircraft. You will need to have a method to catch brake fluid and a plug or cap for the line. Usually there is a snap ring on one end of the cylinder. Remove the snap ring and the internal workings of the cylinder will come out. There are usually two large o-rings, and two smaller o-rings, replace both sets. Make sure the cylinder bore is smooth and clean and that the springs are intact and functional. If all is well and new o-rings are in place, reassemble the cylinder and refill as much as possible before reinstalling in the airplane. Once back in place, bleed the system either by pressure or vacuum (or pumping pedal with a helper).

Service Manual is online under home page tech tab