Replacing brake booster 91 850i automatic

[2001740il]

The brakes work great on this car but the brake booster was leaking a lot of fluid.

So I wanted to replace the brake booster (BB) and not get into the brakes at all.

The mission was to remove the MC from the BB but leaving it attached to the brake lines then remove the brake booster.

1. Remove brake pressure by pumping brakes 20 times etc.
2. Drain the fluid by removing plug at bottom of power steering pump.
3. Remove the pressure regulator (PR) pressure switch. (for Access)
4. Remove pressure PR pipe/hose. (for Access)
5. Remove the two bolts holding MC on, PR passenger side NP. Drivers side use two short 3/8 extensions put together with the one with the 13mm socket being a wobble.
6. Wiggle the MC around, the forward/front end goes into a pocket towards the drivers headlight. The brake lines bend a little NBD. Keep working into the clear pocket area until the MC is away from the BB and you can see the rod from the BB.
7. Remove the BB by going into the cabin and removing the four nuts holding the booster in place and the clip off of the pin at the end of the rod. Remove the pin. The BB is now free to remove.
8. After the BB is out take fork off of rod on cabin side of BB and mount it on replacement BB measure 143mm from estimated cabin mounting wall to end of fork.
9. Put the BB back into car with the bolts lined up move it in place. Put a couple of nuts on so it does not push back out. Place pickle on brake pedal shaft and put the pin back in place and secure with clip. (patience here, or you will bend the pin brake stop light metal tang, both going out and going in)
10. Carefully move the MC back towards front headlight until the BB rod can fit into the small center of the MC. Gently wiggle the MC into place. I then went inside the cabin and gently pressed on the brake pedal. This seemed to drive the BB rod home into the MC and then the job of aligning the holes by twisting the MC was easier and the bolts were able to get a grip from the MC to the BB. Tighten the nuts (extra long 3/8 13mm deep socket and universal joint) in the cabin then then MC bolts to BB.
11. Now reinstall everything you removed and fill up with the green stuff.

Not fun but estimate five hours of continuous effort. I think if you did several you could get it down to about 2-3 really busting butt hours.

[JDP530]

I found the brake master cylinder leaking now on the car I just put on the road a couple of months ago. I shoulda/woulda changed it when I had the manifold off but it looked okay at the time. After removing master (dated 03-91) I noticed the booster leaking so I will remove/rebuild it, but it was pretty darn difficult removing the pipe nut at the back of booster. Tried a crow's foot and just started to round off one corner of the nut so I decided to stop at that point.

I did not have a 17mm flared nut wrench since a box of tools were stolen out of garage. I went by Sears but they are at a loss trying to maintain their inventory so I picked up a cheap set of combination flared nut wrenches at HF.

Wrench is too short to put enough leverage on it so I used a 15mm allen/hex socket on a ratchet to fit into opposite end of wrench and that gave me enough leverage to loosen the nut. There isn't much room in between engine and wheel well to rotate a wrench so using a ratchet made it much easier.

[TM 850i]

Would anyone know where I can buy a new / used one? I'm also looking at buying a kit as well.

[mbarrett635]

Check the parts section - someone has four of them.


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