Just got the car, MY 88 (manufactured 11/87). Very basic trim, driven 280 000kms. It has minor problems but there is also one potentially bigger one.

The brake pedal goes all the way down engine on or off. When it pressed for a longer period, somewhere it starts to hiss and I guess a pump of somekind starts to work. The brake stops and holds the car on a e.g. hill without need to pump.

Discussing with mechanic, this could be a symptom of using a wrong hydraulic oil, ATF instead of CHF. The questions:

-What can be broken? Brake servo? Pressure accumulator? Valve (what valve?)?

-Are any of the parts of the brake servo / assistant system interchangeable with e.g. 735 or E34?