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Thread: hydraulic brake booster unit replacement E32 750

  1. #1
    Join Date
    May 2004

    Default hydraulic brake booster unit replacement E32 750

    written by Patrick C 88 750iL on 2003-01-28 on RF

    Some points in brake booster replacement

    I mainly followed Bentley's procedure to replace the booster. Let me write down some key things I encountered that may help you. Some I neglected to do but if done can save you some misery.

    1) Push brake pedal twenty times or so until it feels hard, to release Pentosin from brake accumulator. Pentosin has to be drained from the reservoir by sucking all up with a turkey baster.

    2) I removed the B+ post and bracket (and heat valve cover) to make room to the booster. Remove battery negative post from below right rear seat first. On 735 you may have more room to work with so you decide what parts around booster needed to be removed. I also loosen the plastic nut on the bracket holding the black cylindrical electrical connectors near the shock tower again to make more room.

    2b) Stuff plastic sheets or newspaper underneath cylinder/booster to catch fluids.

    3) I removed the master cylinder, after loosening and removing the two brake lines that connect to it. And that was after I suck out most of the brake fluid from the brake fluid reservoir.

    3b) ** Here is the part you should try: Try to remove cylinder and booster as one single long piece. Loosen and remove the two Pentosin hoses to the top of booster.

    3c) Then get inside car and remove all lower panels to reveal the brake light switch. You can see two switches interacting with the brake lever if you look up. You should remove the lower (black) brake light switch which is mounted to the bracket by plastic latch near the red ring at the tip. Why? later on you may need to yank on the booster and push against the brake lever, which may break the plastic latch.

    3d) Remove booster's input shaft from brake pedal lever. Loosen and remove the four 13 mm nuts holding the booster. You'll need swivel socket for the top two.

    3e) With brake lines disconnected, Pentosin hoses disconnected, booster disconnected from firewall and brake pedal lever, you can push the master cylinder below the Pentosin reservoir and move the cylinder+booster forward. Tilt the input shaft upwards to take the assembly out of the car.

    3f) Now, outside the car, you can disjoint cylinder and booster and replace with new booster. The two new bolts come with blue loctite but I added some more loctite. Install assemble as a single piece.

    3g) In case you want to probe what's inside the old booster, note that it is spring loaded. Be careful when you loosen the bottom odd-shape screw -- stuffs inside can shoot out to the ceiling. They just missed my face when I naively played with that screw.

    4) I replaced the two grommets between reservoir and master cylinder, even though the old ones were only there for a year.

    5) I jacked up the car, removed all four tires and bleed all four brakes. Left rear wheel first. Yes, it's a pain. However, I didn't need to do anything special to bleed Pentosin. Just half fill the Pentosin reservoir and then run the engine for a while, then push the brake pedal 20 times and refill the reservoir to 1 inch below rim.

    6) Don't put back the lower panels inside car yet. You may need some iterations to adjust the pedal height with a 17-mm wrench. I had problem with "brake light switch circuit" warning but if you take care of the plastic latch on light switch earlier you may be fine. The new booster felt much stiffer when first used. But after just one day brake feel is back to normal.

    Hope that helps.

    Patrick C 88 750iL
    Last edited by shogun; 01-02-2015 at 07:35 AM.

  2. #2
    Join Date
    May 2004


    DIY brake servo booster repair kit and rebuild service

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