Air in brake lines?

I recently noticed that the left brake on my rv-6 was spongy and had 3 times the travel as the right brake. Pumping the left seats left pedal helped but pumping the right seats left pedal helped more. I assumed that air had found its way into the brake line so I bled the left brake and topped off the fluid. Problem fixed for a couple of weeks but now its back. Do I need to rebuild the master cylinders? Should I do all four? Anyone had this problem? No apparent leaks in the system. Thinking about installing the A600 fluid reservoirs. Do I need 4 or 2 of these for a RV-6?

bad cylinder//s

Ok first I don?t know any thing about how you system works with two master cylinders for each side. My -4 had Cleveland master cylinders that lasted 22 years and the right one was getting soft, lots of free play but pumping would help. I bled the system with a pump oil can from the bottom and it was great for about four flight hours, like two weeks. An engineer friend told me that this was a very common problem with what he called an umbrella seal in the master cylinder and I would need to re seal them or replace the cylinder. I replaced both cylinders with some Van?s sells and just bled each one once, no fuss no problems and it has been perfect ever sense. I cant say this is your problem but the symptoms are exactly the same as I had.

If you have the standard setup for right side brakes, you'll only need 2 A-600s. Highly recommended by the way.

A600

What benefit does the A600 provide?

Elimination of the leaky brake reservoir and associated plumbing.

A600 more questions

I take it these are the ACS 600 brake reservoirs.

http://www.aircraftspruce.com/catalog/lgpages/reservoirs.php

Did you mount them directly to the master cylinders then? Anybody got a picture?