2001 XJ brakes not engaging all at the same time

Hello,

Im having a strange issue. When i use the brakes, it feels like the front passenger side seems to engage first. It feels like its slinging around to the left and then the rear driver wheel locks up. I posted something a year or so ago and it was suggested to check/replace the brake lines. Just last week i did the calipers, rotors, pads and lines. My mechanic took the rears apart and adjusted everything. They still do the same thing. Could the master cylinder be be the issue? Its about the only thing left.

Thanks for all help, past and present.

Yes...since everything else has been replaced, it has to be the master cylinder or (if you have one, I'm not sure) the brake proportioning valve.