HELP!!

I have 2006 audi tt. Two weeks ago, i drove it up the highway. Odd burning smell developed and brake pedal started to act odd (hard) at end of trip. I stopped for an appointment. 5 hours later, I drove back down the highway. Same smell. Car became noticibly difficult to drive. I did not realize what the problem was at the time. Kept driving.

I went stright to the mechanic from my appointment. As I pulled in to the shop, the mechanic saw me and yelled that my emergency brakes were on! They weren't.

Apparently all 4 brakes locked up as I was driving on the highway (and stayed locked up on the local roads after I got off the highway). Brakes are now shot.

Mechanic suspects a faulty power booster. If I recall correctly, I think he said the compuer read break pressure error (?). Mechanic is unable to diagnose root cause of problem. Mechanic has been driving my car 1 week and he has been unable to reproduce the lock up. No new fault codes.

Mechanic does not want to replace brakes without knowing root cause. I don;t want to pay for new brakes if there's a risk the brakes will lock up again. So . . the mechanic gave me the car back. I have been driving the car for the past week (just a few miles daily) - no problems at all.

I need to know the root cause right away, so I can get the brakes fixed and peace of mind this won't happen again!!

I tested the power booster by pumping brakes with car off. Pedal went down as I started ignition.

Please help!!!

As an aside, **2 days** before this problem developed, the mechanic replaced a faulty brake sensor switch.