(522-2732) Sinking brake pedal in stationary car at idling speed
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TECHNICAL SERVICE BULLETIN
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Bulletin Nbr:
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522-2732
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Date:
...........
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Maj 2010
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Market:
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all
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Sinking brake pedal in stationary car at idling speed
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This bulletin informs about how to ensure that the brakes are normal or whether action is required.
Saab 9-3 M03- with B207 and Z19DT/DTH/DTR engine with mechanical vacuum pump for the brakes.
The brake pedal sinks when the car is stationary and the foot remains on the pedal.
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The sinking brake pedal is primarily due to the "high reinforcement factor" in the brake servo, which can only be perceived when the car is stationary.
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A large, double brake vacuum servo 9"+9" in combination with an efficient mechanical vacuum pump, that already supplies full vacuum at idling speed is the main cause of noticing the sinking pedal. When the driver e.g. stops at a red light or keeps the foot lightly on the brake pedal, the vacuum level in the brake servo is approx. -0.5 bar underpressure, as the driver has used the vacuum in connection with the last braking procedure before stopping. A mechanical vacuum pump can supply down to -0.98 bar underpressure at idling speed while a car without a vacuum pump can only obtain -0.5 bar underpressure for the same driving situation. When the mechanical vacuum pump lowers the vacuum level from 0.5 to 0.98 bar, more brake servo assistance is obtained and the pedal sinks in pace with the lining and brake caliper being pressed together.
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If the pedal is provoked into sinking to the floor then the hydraulic pressure increases in relation to pedal travel. Full pedal travel means 140 bar hydraulic pressure in the system. ABS intervention on dry asphalt with a fully-laden car normally takes place at 80 bar.
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All cars with mechanical vacuum pump produce the same phenomenon as above. Any differences are due to size of brake servo and the efficiency of the vacuum pump.
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A sinking pedal is more often experienced in cars equipped with automatic transmission, as the automatic transmission's converter remains on and loads at approx. 30-40 Nm in D and R-position respectively.
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Not affected.
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1.
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Stationary car, engine switched off and drained vacuum servo (pump the brake pedal until the servo has been drained): depress the pedal hard. The pedal should feel stiff. If the pedal sinks then there is a risk of air in the brake lines or internal leakage in the master cylinder. For internal leakage, see "Fault diagnosis hydraulic unit TCS/ESP" TSB 521-2476 (US/CA: TSB 521-2481).
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Driving at 50 km/h with the brake pedal gently depressed: if the pedal travel is otherwise normal then it is a sure sign that the sinking brake pedal in a stationary car at idling speed is a result of a raised vacuum level in the mechanical vacuum pump. This acts on the brake servo and accordingly increases the hydraulic pressure on the brake caliper, pads and discs.
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Warranty/Time Information
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