hi, i have a 1999 mercedes 230 slk auto kompressor. after driving for about 20 mins when you press the brake pedal it sticks on and the pedal starts to depress itself and bind on. you have to tap the pedal to release the brakes. i have had the brake master cylinder changed , that didn't cure it. changed the ABS pump and that didn't solve it. my mechanic is now scratching his head. can you help?