I know this is an old post but for anyone that might have the same problem, thedrums are probably bad rusty and that alone will cause the rear brakes to lock-up with very little pressure.

 

Hey Guys,
I just wanted to (belatedly) thank you for your responses to my post. We (hired mechanic - no garage for me to work in inclement weather) ended up cleaning the brake lines by blowing them out, then replacing the master cylinder. The brakes are working perfectly now but will continue to monitor them for future issues.

 

Glad to hear it worked for you and you letting us know it fixed the problem for you. It might help the next guy out.

 

Just a note here guys, my 85 4-runner can sit for about 3 weeks and then only normal brake pedal pressure will lock up the rear brakes, It's only a little rust build-up, after a few stops it is fine.