34 00 Troubleshooting the brake system if the brake pedal fails (530i 2017-2022, 530i xDrive 2017-2022, 530e PHEV 2018-2022, 530e xDrive PHEV 2018-2022, 540i 2017-2022, 540i xDrive 2017-2022, M550i xDrive 2018-2022)

Troubleshooting the brake system if the brake pedal fails or brake fluid level is very low and there is no visually detectable fluid leak 

Test prerequisite: 

• Vehicles with vacuum brake booster (no integrated braking system)
• Customer complaint can be reproduced (if necessary, test drive with the customer)
• Fault memory entries in DSC control unit: 
  • Possible, brake fluid level is very low
• Other fault memories in the DSC control unit must be remedied beforehand
• Vehicle/brake system has already been bled → The problem could not be solved by venting.

Problem analysis/troubleshooting: 

1. Check the brake fluid level in the expansion tank (take into account sinking due to wear of the brake pads/discs).

   If the fluid level is implausibly low, check the tightness of all screw connections, hoses and lines of the hydraulic brake system.

   If there is liquid leakage on a screw connection, release it slightly, if necessary align the line/hose free of tension (the union nut must be screwed in by hand up to the limit position without great resistance) and then tighten the screw connection to the NOMINAL TORQUE .

   Check the screw connection again for tightness (apply 30 sec high brake pressure). If fluid continues to leak, check the sealing surfaces for damage/contamination. Exchange/clean the component if damaged/contaminated. If the leak is not directly in the area of a screw connection, exchange the leaky hose/line.

   If the brake fluid level is too low (warning light on permanently), there are no indications of leakage in the screw connection/hose/line, detach the vacuum line off the brake booster. Use an endoscope to check whether brake fluid has got into the brake booster.

   If so, replace tandem brake master cylinder and brake booster.

2. Place vehicle onto vehicle lift. Insert gearshift lever in neutral or gearbox level N in automatic vehicles. Raise the vehicle a little until the wheels lift off the ground and, when the brake is not applied, can be turned slightly by hand (parking brake released).

   Pinpoint the possible fault cause as follows: 

   1. Switch off the engine and leave it switched off during the test.
   2. Press the brake pedal several times until it feels constantly hard (reduce the vacuum in the brake booster)
   3. Apply constant moderate pedal force to the brake pedal (brake pedal sinks slowly in the event of a fault). At the same time check on one side of the vehicle whether a slight braking effect is achieved on the front or rear wheel. Braking effect must still be present on both axles on one side of the vehicle while it is sinking.
   4. Apply the brake pedal with constant effort until it does not sink further. First keep pedal force as constant as possible.
   5. After a short waiting period (5-10 sec) the pedal force can be significantly increased as much as desired.

Inner leak of tandem brake master cylinder: 

The tandem brake master cylinder has two independent brake circuits. If there is an internal leakage in one of the two brake circuits in the tandem brake master cylinder, either the front wheel or the rear wheel can be rotated.

Exhaust valve in the DSC control unit is leaking: 

If the valve in the DSC hydraulics leaks, neither of the two wheels can be turned when the pedal is depressed and the brake pedal force is high.

As an alternative to testing by hand on a vehicle lift, the wheel-specific braking effect can also be checked on a brake test stand.