I have seen this a couple times as a result of a faulty
calipre.  The problem is one of the calipres is not fully
releasing resulting in those pads being warmer than the
other side as you drive. When you apply the brakes, the
warmer pads are already up to optimum braking temperature
intead of needing a couple of turns to get there.  The only
fix I have seen is to replace both calipres because you
basically have no way of determining whether the problem is
a tight or loose piston or a bad casting.  Also, the brake
flex lines to the wheels can do this if the inner liner
separates resulting in a "check valve" effect retaining
pressure on one of the brakes even when released.  The
result is the same.  I have seen this occur right after a
brake job where the tech either dropped the calipre  while
it was still attached to the hose or simply left the calipre
hanging by the hose instead of tying it up with a wire to
support it while the brakes are being service as they should
be.  I suspect the calipre is your particular problem since
you say it needs to be driven a bit before the problem
occurs.

car pulls hard left.  I have had it back to the shop 4 times.  New pads,
new caliphers, new rotors, new seals, repack bearings, new wheel seals,
cleaned and adjusted rear brakes.  Car can be driven a few miles and it
will not pull.  All of a sudden when braking, it will pull.  If I slam on
brakes it doesn't seem to pull.  When I brake normally, i.e., traffic
light, stop sign, etc., it then pulls severely.  Anybody have any
suggestions?  Thank you.