Later 740/940 cars have a Hella black plastic vacuum switch at the brake pedal. This has a small square section O-ring that seals the vacuum when the pedal is pressed, signalling the cruise mechanism to decouple. If that O-ring leaks, you'll have a vacuum leak in your system. The Hella switch (Volvo p/n 1377159) is no longer available. You can replace it with a much more expensive VW equivalent (VW p/n 811907343B for about $60). Or you can disassemble the switch and replace the O-ring with a Size 008 Quad ring (size AS568-008 (ID=4.65mm; CS=1.7mm) from ORingsandMore.com. To do so:
1. Remove the cap by pressing soft aluminum wire under the four locking tangs with a screwdriver to gently lift them up and remove the top of the switch.
2. Pull out the center shaft, the brass contact and the spring.
3. Using a hook, remove the O-ring in the base of the switch.
4. Grease a new O-ring with synthetic grease, place it on the shaft, insert the contact on the square cross section, then the spring, and reinsert it.
5. Place the cap on and press home to lock the tangs.
Works fine. The quad or square section O-ring will last much longer than a normal round cross section ring since all the wear is concentrated on the inner diameter.
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See the 700/900 "FAQ" at the menu bar top screen left side.