I did a full rear break job not too long ago, maybe 15,000 miles max, replaced shoes and rotors and my driver side cylinder because it was leaking. Now the same side cylinder is leaking any idea what would cause this to happen already? does it have anything to do with the Renix ABS? not even sure if i have this but heard it was junk.

215,000 miles still killing it!

 

What model year? With "Renix ABS" i assume late 80's to '91. Also did you actually confirm it's the wheel cylinder leaking and not the line going to it? If it isn't the line it could just be you got a crappy wheel cylinder, so i would go ahead and replace it with a high quality one (OEM spec) if you confirm it is the cylinder leaking.

If you like to live dangerously you can pull the cylinder apart and check the seals, maybe even rebuild it.