Low speed braking
 

Another week another issue with the X

At low speed the brake pedal goes solid and feels like it has no stopping ability. My understanding is that it’s the brake servo unit (34336760461)

Hoping someone can validate this for me and then advise whether I should also change the master cylinder at the same time.

Thanks all

The vacuum pump is where I'd look. For some reason BMW thought the 8 cylinder engine couldn't make enough vacuum so..... let's just say when mine failed after a 4 hour highway trip that in town traffic made my seat change into a traffic cone [emoji28]

If you hear a gurgling like water behind the dash it 100% the pump.

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Low speed braking
 

Thanks for the info - sorry this is a total noob question but I had no idea there was a brake vacuum pump on these, where is it located/what does it look like? If I google I’m seeing this thing - which I know I changed the o rings in a few years back when I had it off for the valve stem seal replacement, had no idea it had anything to do with the brake system though https://uploads.tapatalk-cdn.com/202...e74d33394a.jpg


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Here's the part numbers https://uploads.tapatalk-cdn.com/202...78815c8c42.jpg

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With Valvetronic operating correctly the throttle valve is open so there's not much vacuum in the intake manifold.

I would first check the vacuum pump is producing vacuum and also check the non-return valve on the hose connection. It seems that the valve is not available separately but comes with the hose.

https://www.realoem.com/bmw/enUS/sho...diagId=34_1350

So I started by replacing the hose and clamps as the hose was rock hard no change, changed the pump unit entirely and still no change. I press the brake, hear a whoosh of air and then the pedal goes hard with no brake force.

Any ideas where I should look next?

Thanks


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Check valve?

If the brake booster "whooshes" air that suggests a vacuum leak in the diaphragm side past the valve associated with the brake.

Run the car to build vacuum, turn off the car and have somebody else press the brake while your are under the hood listening to the brake booster to figure out where the air whoosh is coming from.

I have these same symptoms. Vac pump pulls and pulses. I do hear a whoosh sound. Does this point to booster?
Thanks!

I would guess so. I’m waiting for a new booster to arrive. There is very little that can go wrong with the vac pump itself outside of the o rings. The cheapest place to start would be the rubber hoses at each end of the vacuum hose, mine were definitely perished but I’d just be guessing at anything else.

I would suggest following Andrew’s instructions above