Need an expert!  My brakes become inconsistent after driving for 40 minutes or 40 miles.  They work perfect at first.  I have had the whole system replaced and went to synthetic fluid.  When the car has been driven for a distance, the left rear brake does not release properly.  It drags when you stop and go again.  Actually left rear and fronts have all done this but normally, left rear is the first to grab and slow release.  This occurs during soft braking at very slow speeds.  I am thinking I have something besides the brake system causing the problem, but what?  We have rechecked the new calipers, new lines, new pads, etc, and nothing visable to cause problem.  We have re bled the lines twice.  Your guidance will be greatly appreciated!

You may not have gotten all the old brake fluid out. DOT 3 or 4 brake fluid does not mix with synthetic and causes problems.

Either get rid of the synthetic and go back to DOT 3, or make sure ALL the old fluid and moisture is out of the system. Either way you need to completely drain the fluid and make completely sure it's free of moisture. That includes the seals. Synthetic fluid and moisture do not get along under any circumstances.

For a variety of reasons most, if not all performance cars specifically say to avoid using synthetic brake fluid.

Either a bad flex line, master cyl. or proportioning valve. It can also be a steel line that has been bent/kinked/flattened somehow.

Also, check to make sure the e-brake(s) are not dragging. Even if the cable itself is loose, the adjutment could be too tight, and create a drag that will get worse as the e-brake shoes heat up, and expand.