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I pulled the MC and noticed the fluid at the bottom of the booster. I would assume that it has to come from the MC but the MC looks dry at the back and no sign of a leak there?
Also...has anyone done a "bench bleed " on the MC to assist in bleeding when the whole brake system is dry?

just rebuilt the master cylinder on the 528i and I found the same thing. It struck me as odd that every 6 months or so fluid was missing from the reservoir. The MC or the reservoir did not have a visible leak and I could only deduce that the seals between the master cylinder and the reservoir had finally given up. Using a vacuum tool I pulled out about half a liter of fluid from the booster.

Check underneath the carpet on the driver's side as you may have a pool of brake fluid there as well. The foam mat behind the driver's seat was melted in place.

I can recall reading somewhere that, as the brake booster is run off the engine vaccum, then fluid that builds up in the booster eventually gets sucked into the engine and is burnt off - giving a rather unusual smell to the exhaust, and mysterious loss of brake fluid!

I have a car that had a brake failure - removing the booster and pulling it apart revealed it full of fluid, fortunately I had another one, so simply replaced it - a rebuild was going to cost $550


It appeared to me that the internal seals to the cylinder have failed leading to a booster getting filled up with fluid

note on RHD we have 2 x remote servers mounted on left side fender.

Ive had a go at pulling the servo apart - but don't have the tool to remove the internal screw - which I presume give access to the internal "plunger"
.

front cover - removed by move out outer crimped edge on the cover.
diaphram moves in and out easily enough - but has no visible way of removing it from this side.


vent housing held on by screws - came apart easily enough




revealing the innards
airvalve, spring and diaphram


behind diaphram is a small screwed on cap with 2 holes for a special tool - punch didn't work - Im hoping by removing this screw cap, that I get access to a screw or something that will enable plunger to be fully withdrawn.

Hello to The Land Down Under from the Great White North
Nice to see someone still repairing E12's and keeping them on the road. Thanks for the pics in the brake booster.I think I will get to that eventually but I have another brake issue I am trying to solve first. I decided to replace the brake lines from a 530i 03/76 Manual to a 530i Auto 09/76. The Manual had a "brake pressure switch" which I left inline when I installed the line in the Auto car. It seems that this "brake pressure switch" was done away with on the later Auto models? so I probably should have left it off?
Now when I go to bleed the brakes starting at the rear caliper I can't get any brake fluid to flow back there and I think it is because this brake pressure switch is blocking the flow? Wonder if they make some "M10 brake line connectors" that I could connect the brake line together when I remove that switch which I apparently dont need for the Auto car? Any thoughts on this? Wonder why BMW discontinued the switch?

PS Both cars have the brake pressure "regulator" that is in between the Master Cyl and the brake line that goes to the rear brake calipers.
Karl

Further to my post of dismantling the brake booster - in my case we have 2 x boosters for RHD cars, that are remote to the brake pedal.

Anyway a friend showed me how to dismantle the booster further.

At the end of the push rod, is a rubber band - remove the rubber band with a dull pick, put aside - or put in a bag.




Underneath this is a metal spring/sleeve , lever that off with a pick. thus exposing a metal pin - approx 2-3mm dia , use a small punch to push that out - a bit of lube helps.
rubber and metal sleeve


pushrod end with hole for pin- now removed


once pin is out, pull diaphragm off the push rod.

underneath the diaphragm is a large spring - that can simple be taken out.



now we have the inside of the canister exposed - at base is 3 bolts - undo bolts and remove the cover plate, this also allows the canister to be removed from the brake housing.



canister now removed too.


gently remove the seal/cap- I simple tapped it with a punch lightly which seemed to upset it enough to release, then using a pick pulled it out gently - friction seems to be the only thing keeping it in. ( see note at end)



now you need something to pull out the rod , I used a bent bit of wire in the pin hole at the end. I clamped the housing to a vice, then gently tapped the seal and whole thing kind slipped out with requiring too much force.

brake housing in vice, and gently tapping the seals whilst pulling the push rod enabled the whole thing to slide out


thus we get the push rod with all the seals still connected.





NOTE: BTW I sprayed some WD40 in the the housing orifices before I started removing push rod - to help unstick anything - I also gave the pushrod a good hard push to release the seals from the housing bore - before attempting to push the push rod out - my understand is that the pushrod ought to move rather freely in and out - mine was quite stuck (outwards) and didn't initially move freely.

Next step is checking the housing bore - ie is not corroded or pitted, then getting new seals

Evidently Garage 02 in Melbourne has a service kit for these.

https://garage02.com.au/product/brake-b ... build-kit/

garyinwestoz wrote: Thu Apr 06, 2023 3:10 am Further to my post of dismantling the brake booster - in my case we have 2 x boosters for RHD cars, that are remote to the brake pedal.

Wow. Talk about not looking like fun. I've played with the hyrdroboosters on E28s and they're a little bit of a PITA, but nothing like you have to deal with. Does that happen a lot on RHD factory conversions or just once in a while? Looks like they made it work, but...

Mike W. wrote: Sat Apr 08, 2023 9:35 pm

Wow. <snip> Does that happen a lot on RHD factory conversions or just once in a while? Looks like they made it work, but...

I think all the BMW's we got prior to the E30 and E28 had twin brake boosters - certainly E3/E9/E12 do .
There are some people looking at changing them to single boosters.