Servo cylinder Volvo 940/960 with ABS

A hard brake pedal, a hissing sound when braking, or a pedal that feels sluggish while the engine is running can point to vacuum loss in the brake servo or a stiff non-return valve. On 940/960 with ABS, the servo seal and the vacuum hose's non-return valve should be checked before condemning the whole unit — a leaking non-return valve often produces the same symptoms as a faulty servo.