[JasH]

Originally Posted by medicj

On a slightly different topic, if my brake failed should i put my gear to park and turn off the engine at a time of emergency?

I would instead press super hard on the brake pedal.

If that fails completely, put in Sport mode and lift off and apply the parking brake.


Failing that, look for a soft crash & bend knees and elbows. Up a verge or scrape a crash barrier.


Applies to any such car, not just i8.

[Ionicsilveri8]

So I did a search across the forum for brakes in the i8 and came across this. My problem is that I have experienced three times now a problem with the brakes.

First time, I was approaching a T junction and applied the brakes as normal but the car failed to slow, it did slow but not as I was used to. It was as if the drive was still engaged and was “pushing” the car forward. So normally, the brakes would just stop the momentum of the car but on this occasion it was as if I was fighting against the momentum plus the car trying to continue to drive. Does that make sense?

The car was relatively new to me so I just thought it was a glitch however it happened again a few weeks ago, this time approaching traffic lights on red with a number of cars waiting. I did manage to stop but again, the sensation I had was that I was fighting against drive.

On both occasions I was in sport mode, no rain, daylight, no traction light came on and no error messages. I took it to the dealer and they have just given it back - it needed a 3 year service which they did and at the same time also tried to diagnose the braking fault.

After two days they are stumped. They said that they could continue driving it to experience the problem themselves but it has only happened three times in 10,000 miles (the third time was minor) so they would need a lot of test driving to duplicate the problem. I now have the car back and have been told that if it happens again, I must call BMW Assist (as long as I’m not wrapped around a tree)!

Has ANYONE ever experienced this so that I can suggest a solution? I am told that they kept it overnight so that BMW UK could interrogate the car remotely to try to diagnose a fault. Not sure that I believed that!

Any comments appreciated.

[Nachbar]

A friend of mine had this, but with the steering support (540d). It completely failed while he drove fast in a corner. He is a strong guy, but had a real hard fight, keeping the car on the road. The papers of what the dealer had to work and parts which haf to replaced is 3 pages long. It did not occure again since then, but He doesn't feel comfortable driving it now. Well.. most of the time his girlfriend drives it

[Ionicsilveri8]

Originally Posted by Nachbar

A friend of mine had this, but with the steering support (540d). It completely failed while he drove fast in a corner. He is a strong guy, but had a real hard fight, keeping the car on the road. The papers of what the dealer had to work and parts which haf to replaced is 3 pages long. It did not occure again since then, but He doesn't feel comfortable driving it now. Well.. most of the time his girlfriend drives it

Interesting, but not really related to my issue? Anyone?

[djbmw1]

I'm reviving an old thread in the event that anyone wishes to gain further insight into this as a failure in the braking system is CRITICAL and needs to be dealt with IMMEDIATELY before continuing to drive.

Your Brake Booster has a bladder inside of it (rubber membrane) that, in order to work, must remain under vacuum. This vacuum is achieved via a vacuum pump. The vacuum pump is connected to the brake booster via a vacuum hose with a check valve/one way valve somewhere in line (usually right at the end of the hose, closest to the brake booster).

Now, if your vacuum pump starts to crap out, or the vacuum line has a split in it, or the check valve craps the bed or, if you have a different model of BMW whereby the vacuum pump is oil cooled and the internal seals fail, thereby allowing oil to make it's way into the vacuum line, these will all cause the symptoms that OP had.

Essentially, the brake pedal will feel much harder than normal and you will NOT have fully assisted braking. This is ESPECIALLY true at low RPM's for those that have a BMW model where the vacuum pump is powered by the engine (vs an electric vacuum pump like in the i8).

To trouble shoot, remove the vacuum line from the brake booster and hook up a vacuum gauge to read what the vacuum pump is doing and if it's within spec (look up the specs for your pump). Then, remove the other end of the hose (so the hose is disconnected at both ends) and apply a vacuum with your gauge at the end that connects to the vacuum pump - see if you get 18 to 19 inches of mercury (HgV) and see if it holds for 5 min.

For models that use a mechanical vacuum pump (not electric like the i8), see if there's oil in your vacuum hose - this will indicate failing seals on your pump.

Finally, if your vacuum pump is sucking the right amount... and your hose + check valve isn't leaking... then the bladder within your brake booster likely has a leak.

You can also test this by running the car for 30 seconds and, without pressing the brake, turn off the car in a QUIET location. Now, pump the brake pedal a few times while CAREFULLY listening to the Brake booster. If you hear ANY type of hissing, or air (or even liquid) then you know you have a leak/tear.

[Coastali8]

Standard power braking assist on a gasoline powered vehicle uses vacuum from the engine intake manifold. There is no vacuum pump.
The i8 does have an electric vacuum pump. Sometimes things fail. Then you are left with no vacuum assist. Just press harder on the brake pedal. The braking system is still intact, don’t be a weenie just exercise your leg to stop.

[P.K.]

When a brake failure occurs by any means the first instinct is to push that peddle with all your might right through the floor. I have personally experienced both the stiff and the peddle goes to the floor symptom. I have been able to snap my car into a sideways slide to a somewhat controlled stop. All that said, the thread has come to DIY diagnosis - we all want to know what happened in this "fail safe" system. I previously suggested an ABS failure but since the symptom has gone from completely stiff peddle (locked out) to on the floor coasting there is much more to consider. There is a simple way to check the boost system. First, before turning the car on, press the brake peddle many times to deplete any vacuum in the booster. Next, while holding your foot on the peddle with moderate pressure, turn the car on. The brake peddle should go down an incremental amount indicating that the booster is assisting with brake force.Release the peddle, wait a moment, then shut the car off. Wait several minutes.The vacuum in the brake booster should hold enough vacuum for several applications. After each successive application the peddle should get harder to press until the reserve is depleted. Failing this test can be the result of any of the suggested issues. This might at least get the focus on other parts of the system
I have seen ABS failures that don't exhibit a code but are there non the less.

[stressdoc]

In response to what to do if brakes fail. Use emergency brake. Do not turn car off. You can downshift and eventually put tran into park, but doing that at speed will result in override preventing wheels from locking up.

[X5Pye]

Originally Posted by djbmw1

I'm reviving an old thread in the event that anyone wishes to gain further insight into this as a failure in the braking system is CRITICAL and needs to be dealt with IMMEDIATELY before continuing to drive.

Your Brake Booster has a bladder inside of it (rubber membrane) that, in order to work, must remain under vacuum. This vacuum is achieved via a vacuum pump. The vacuum pump is connected to the brake booster via a vacuum hose with a check valve/one way valve somewhere in line (usually right at the end of the hose, closest to the brake booster).

Now, if your vacuum pump starts to crap out, or the vacuum line has a split in it, or the check valve craps the bed or, if you have a different model of BMW whereby the vacuum pump is oil cooled and the internal seals fail, thereby allowing oil to make it's way into the vacuum line, these will all cause the symptoms that OP had.

Essentially, the brake pedal will feel much harder than normal and you will NOT have fully assisted braking. This is ESPECIALLY true at low RPM's for those that have a BMW model where the vacuum pump is powered by the engine (vs an electric vacuum pump like in the i8).

To trouble shoot, remove the vacuum line from the brake booster and hook up a vacuum gauge to read what the vacuum pump is doing and if it's within spec (look up the specs for your pump). Then, remove the other end of the hose (so the hose is disconnected at both ends) and apply a vacuum with your gauge at the end that connects to the vacuum pump - see if you get 18 to 19 inches of mercury (HgV) and see if it holds for 5 min.

For models that use a mechanical vacuum pump (not electric like the i8), see if there's oil in your vacuum hose - this will indicate failing seals on your pump.

Finally, if your vacuum pump is sucking the right amount... and your hose + check valve isn't leaking... then the bladder within your brake booster likely has a leak.

You can also test this by running the car for 30 seconds and, without pressing the brake, turn off the car in a QUIET location. Now, pump the brake pedal a few times while CAREFULLY listening to the Brake booster. If you hear ANY type of hissing, or air (or even liquid) then you know you have a leak/tear.

Thanks for reviving this thread. I have a 2012 X5d and after having my rotors, sensors and pads replaced, I noticed the brake pedal was at the top when started and would not drop slightly as previous. I assumed it was due to new rotors and pads. I was wrong. Then I had my X5 in for service and the mechanic flagged the vacuum pump as NG and needed replacement. The owner looked at it and said it was OK, that there was a slight oil leak around the vacuum pump, but it should be OK for now. I wondered if the issue had something to do with the vacuum line being disturbed and that the owner saw the issue and fixed it. I drove it on a 5000 km trip and have been back for 2 weeks and it hasn't happened again. The cost of replacing the vacuum pump would be $700CAD for parts plus labor.

Question: Am I out of the woods on this?

I plan to run the diagnostic when I can but since the mentioned service visit, I have had zero firm brake pedal response when starting.