1995 Toyota Landcruiser • 290,000 miles

What would cause the front left brakes to come on and lock up without depressing the brake pedal?
I have replaced the pads checked all pistons are free and fully bled the system, all brake lines and proportioning valve appear fine with no leaks or kinks. Only the one wheel is affected and when it is locked up I turn the engine off and after a period of time it goes back to normal, untill it locks up again.
January 2, 2013.

Since last posting both front brakes are applying themselves whilst driving with no pedal contact whatsoever, have checked both front caliper operations and carried out complete brake fluid flush, have now filled with a Toyota approved brake fluid and bled all four calipers, had no issues for a few weeks, travelled roughly 200km's and now all brakes are applying themselves. I have absolutely no ideas left and can not find any reason as to how or why the brakes are coming on themselves.

Apr 23, 2013.