The accumulation of moisture at major connectors and control units can cause various ABS/PDAS faults due to corrosion.
Resources for the Repair and Maintenance of your 1991 Porsche 911 Turbo
1991 Porsche 911 Turbo
1991 Porsche 911 Turbo Problems
The brake warning display and warning gong can activate, typically after the first start of the day. The warning light/gong might stop after idling for a short time. This fault is usually caused by low stored hydraulic pressure due to a failed high pressure accumulator.
An accumulation of small air leaks at various locations around the intake manifold/intercooler area can cause performance problems.
An external fluid leak may develop at the pressure switch; fluid will be evident in the trunk and the reservoir level will drop.
The hydraulic pump relay can fail, causing the pump to run continuously, which leads to hydraulic pump failure due to overheating. Incorrect pressures will cause a low brake pedal or a high pedal effort with the possibility of other strange symptoms.