Brake Lock-up every 7 Km

Symptom: Brakes locking up after 7 km.

This 2003 Hyundai Elantra came in for brake lock-up problem. Some other shop had already worked on and according with them it was fixed.

It had a new right front caliper and both front steel brake lines are replaced. The customer was almost in tears because of the problem wasn't fixed with the other shop.
We know the shop, so we called them and asked about this car, they said it should be OK now after they've bled the air out of the brake system.
Customer said that if she drove about 7 km, the the brake does not release, feel like locking up. So, we road tested for the said 7 km, and lift it up on the hoist, sure enough the left front brake was locking up, we couldn't even rotate it with both hands. We loosened the bleeder screw, then it freed up. The right front brake and both rear brakes were not locking up. We re-road tested again to confirm the problem, but this time right front brake also was not releasing properly. After further diagnostics, we found that the brake light switch at the brake master cylinder was loose. This could contribute to the problem, because the loose brake light switch can push the master cylinder rod and cause the brake pedal to not raise properly.

We adjusted the switch position and the push rod's free play properly and re-road tested again. It was much better, but still both front brakes did not release properly.  This time we could rotate the wheel with both hands. So it was better than before, but not a 100 % fix.

We suspected the master cylinder has some clogging in its internal passage, but one of our automotive seminar instructors suggested to disconnect  the vacuum brake booster and see if it corrected the problem.  It apparently is a pretty common problem found with some Ford vehicles.  We road tested another 7 km, and we confirmed the brake released when booster was disconnected. We then ordered a used vacuum booster and master cylinder together.  We replaced the booster first but it did not fix it, so then we replaced the master cylinder --then it was fixed. We road tested another10 km, finally, to confirm the fix.

Side notes: This car has some more issues: the clutch is beginning to slip, the right front tire belt shifted, and left rear tire has uneven wear and causing some noise. The brake line that other shop fixed, was not done very well. We suggested that we should fix it properly in the near future.
In the end,  the customer is very happy that the main problem has been fixed for good.

P/S, The customer called us back a week later and said that the other shop paid her $800 back. I would like to respect and praise this shop's proper judgment & compensation.

Above: The new brake line was slightly rubbing on the inner wheel when full turn cornering.