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Scary brake locking on freeway

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I took out my 1999 996 recently on the freeway and about 20 miles into the trip all 4 brakes engaged forcing me to limp off the freeway. No warning lights were visible on the dash (abs/psm etc).  Took it to a well respected indie mechanic and he resurfaced the rotors and did a complete brake job with new fluid however the following day the brakes locked again after a short 15 mile trip.

Anyone have an idea on what could be causing this behavior? 

Note that I had the brake booster replaced about 18 months ago because the brake pedal was not returning to it's original position. Indie didn't find anything suspect with the new brake booster.

I have the following aftermarket parts installed:

Dectane led rear brake lights

Wheel spacers

Fister exhaust

Any help would be greatly appreciated as it's been at the mechanics for over a month now and he is still trying to diagnose the problem.

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Without actually seeing the car, I would start by having the control network for the ABS/PSM checked out.  There are a lot of valves and controls in that system, which are susceptible to build up from corrosion and can jam or malfunction.  I would also try pulling the vacuum line to the brake booster when the brakes are locked and see if that releases them.

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Just wanted to say thank you to everyone that responded and provide an update.

Looks like the brake booster was installed incorrectly by a previous mechanic and the pin was not correctly aligned. I didn't notice this for quite some time because the car is not a daily driver and has not been taken on any extended trips until recently. The 30+ mile extended trip was sufficient to cause the brakes to engage. 

The mechanic replaced a few o-rings,adjusted the brake booster, and took several 30+ mile trips to see if he could reproduce the issue. Just got home and looks like we are good for now. 

Again, thank you to everyone that responded.

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