The Volvo 960 was first introduced in 1990 as a 1991 model. It was a replacement for the 760 series. Production of the 960 ended in 1998, with the 1998 model year vehicles renamed as the Volvo S90/Volvo V90.
The following article discusses Volvo 960 cruise control system Diagnosis, Testing and Troubleshooting. The cruise control system consists of main switch, brake lever, control unit, vacuum pump and regulator, vacuum servo. Cruise control main switch is located at end of directional signal lever. Selector switch has 3 positions, OFF, ON and RESUME. SET button is at tip of lever. Brake valve is located on the pedal assembly. Two types of valves are used, but their function is identical. Valves are interchangeable. Older type of valve is adjustable because plunger sleeve is threaded, allowing valve to be adjusted in or out for necessary disengage position. New type of valve adjusts using small plastic catches which hook on to plunger, holding it in place.
In this article you will find also procedures for adjusting the brake valve and diagnosis/testing the faults. Here is a quotation from this section:
Cruise control diagnostic system does not have a diagnostic hookup, but only a short test lead wire next to control unit. Connect a jumper wire between test lead on control unit and wire on diagnostic unit.
Remove vacuum hose and throttle cable at vacuum servo. Use care not to damage rubber seals. Unscrew vacuum servo from retaining bracket. To install, reverse removal procedure. Ensure throttle cable is taut at idle. Cable must not affect position of throttle.