Brake MC - whats the inner half with a sensor on it ?

[thyrsted2@olsy.dk (Kenn Thyrsted)]

Persuing my brake problem when weather is hot or the radiator fan has run
for a number of times, i discovered that cooling the inner half of the MC
(the half bolted on the firewall) would promptly result in a firm pedal, and
restore braking to app. normal.

This inner half, bolted on the MC in one end and the FW in the other, has
some sort of sensor in it, maybe the brake-warning-red-triangle-triple-beep
sensor. What is this inner half ?

Any help would be appreciated.

Kenn Thyrsted
'87 200 TQAvant