How to replace the “brake bomb” in your E31 BMW 840i, 840Ci, 850i, 850Ci or 850CSi
What does the brake pressure accumulator do? Well, normally your brake booster works based on pressure built up by the power steering pump. But that pressure is not quite enough for sudden hard braking, so BMW added the brake pressure accumulator to… well, accumulate pressure (duh!) for hard braking.
Symptoms of a failing brake pressure accumulator (brake bomb): the brake pedal feels hard upon sudden braking. If you hold your foot on the brake, it gradually gets softer and the car starts braking better, but the initial jab on the brakes feels like the pedal is not doing much. There’s also a test you can do with the car stationary and the engine off: stomp on the brake pedal about 20 or more times. The pedal is supposed to feel relatively soft (such as during normal driving) for the first 15-20 times and become hard after that. If it feels hard from the beginning, or after only a few presses, the brake bomb is bad – that means it can’t accumulate enough pressure.
E31 brake pressure accumulator replacement procedure:
- Watch the video below – it goes through everything. Then follow the steps here 🙂
- With the engine off, stomp on the brakes 20 times to release whatever pressure there may be in the accumulator.
- Jack up the left front of the car (support it by a jack stand and whatever other safety measures you see fit – e.g. I always put a wheel under the car in case the jack stand fails or slips – you can see that in the video).
- Remove the front left wheel (apparently both LHD and RHD cars have the brake bomb there).
- Remove the inner access panel in the front part of the wheel well (again, see video if this is unclear).
- Put some paper towels around the brake bomb to catch dripping fluid. You don’t need a basin or any large container – it won’t pour, it’ll just drip a little.
- Use “raptor” pliers such as the Knipex 8741250 to undo the nuts that hold the hydraulic lines going into the brake pressure accumulator. I rounded off mine with a regular wrench and even with a flare nut wrench. Having these raptor pliers is highly recommended… and pretty much required. See the video for visual proof.
- Lift up the pipes out of their holes in the brake bomb, undo the nuts on the front and rear mounts and then push up the back of the bomb to lift it out. See video for details.
- Transfer the rubber bushings from the mounts of the old accumulator to the new one (or install new bushings on the new one).
- Put in the new brake pressure accumulator, install the hydraulic lines in reverse order.
- Check your fluid level and top it up such that you can just barely see fluid to the bottom of the screen at the top of the reservoir.
- Start the engine and let it run for a short time, then shut it off and check for leaks at the brake pressure accumulator.
- Check the fluid level again – it has most likely gone down a bit due to filling up the brake bomb and lines. Top up again.
- Run the engine again, check for leaks, and if everything is fine, reinstall the access panel and the wheel and lower the car to the ground.
- Take the car for a careful drive, test some hard braking and make sure everything works fine.
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