[kmaforty]

Looking for help to diagnose a brake issue on my 06 330i manual. I've searched for similar comments on other threads, but have not been able to find anything.

Issue: During a panic stop or hard brake event, the brake pedal is hard and does not plunge. Worst, there is little brake force and have to smash the pedal to slow down. This only happens when the brakes are stabbed or pushed hard and seems worse when the vehicle has low vacuum (neutral or low RPM).

I have no issues during normal driving. Coming to a slow gradual stop using light to medium pedal force, pedal effort builds up and is firm and consistent and brake force is good. Stopping power is smooth and controlled.

I have tried the usual checks for booster leaks:
1- Brake booster: pump brakes, hold, turn off engine, pedal drops. check.
2 - check valve: engine on, hold brake, pedal does not rise. check.
3 - brake booster: run engine, shut off, pump brake. pressure builds. check.

I changed the fluid and no improvement. Pads and rotors also look OK. Any suggestions?

[bluewater328]

Wow, got me. You wrote down my exact list. ABS/Hyrdo?

[PerfectAce]

Originally Posted by kmaforty

Issue: During a panic stop or hard brake event, the brake pedal is hard and does not plunge.

This is by design. Part of the Dynamic Stabilty Control system on BMWs is the Brake Readiness system. The system is active at speeds above 70 km/h.

From BMW: "If the DSC system notices that the accelerator pedal is released quickly, the brakes are immediately pretensioned, to shorten the brake apply response time. To accomplish this task the DSC system generates a low braking pressure by applying a PWM signal to the solenoid valves, without creating any measurable deceleration of the vehicle. By applying a small amount of brake pressure the working clearance between the brake pads and brake disc is reduced. If the brakes are not applied within a certain time, the brake pressure that was applied is reduced."

Your brake pedal does not appreciably plunge during this situation because the car has already moved the pads as close to the rotors as they can get without slowing the vehicle. As you stomp on the brake pedal, it's not going to move much further because the pads have little to no travel left and you can't compress your brake fluid.

Do you really have "no brakes" during sudden braking from high speed? Or is the thread title just an exaggeration? Because I would think that if you really had no brakes in a panic stop situation, your title would be more along the lines of: "Totaled my E90 after T-boning a Buick". The brake pedal feel is markedly different when the Brake Readiness system is triggered. Are you sure you're not just mistaking this feature for a problem?

[juld0zer]

There are some inconsistencies:
No E90 uses manifold vacuum to operate the brake booster. So the only time an E90 has a *low* vacuum situation is when the engine is off. This is because they all have engine driven vacuum pumps. They produce plenty of vacuum even at idle and of course will produce more vacuum with a higher engine speed. It will always be a challenge to evacuate 2L of air from a booster instantly.

I don't agree with your methods of testing the booster for leaks but perhaps you may not have explained your method well.

PerfectAce - the brake readiness system (pretensioning) should produce very sharp brakes if it was activated. On a manual 330i it should be quite easy to trigger it by accelerating hard in 4th gear, blip and downshift into 3rd and quickly apply the brake gently as if you were trying to make a light on a bend. It will surprise you as your foot muscle memory is used to pushing a certain distance to achieve a certain amount of reaction.

OP, to me, it sounds like a car with a manifold vacuum source for its booster, with a non functional check valve.

Inspect the vacuum line from the vacuum pump (underneath the intake manifold) to the booster.
Check for chafed plastic pipe especially at the anchor clip near the left strut tower.
Check the tee and hose for the exhaust flap. Have you done the 'golf tee mod' to your exhaust flap?
Check the check valve - run the engine for a few mins and then shut it odd, come back in 4hrs and test the brake pedal. Is it hard/shorter in travel than normal? It should feel the same as if the engine running.
You can remove the check valve and blow/suck test it or smoke test it to verify function. The grommet can sometimes wear/shrink and leak slowly also. Check for presence of motor oil in the vacuum line and check valve.

What repairs did you do recently?

[opjose]

The pretensioning system makes the braking easier to achieve not worst.

It merely reduces the amount you have to push before they become effective which is not what the OP described.

They make the brakes feel more responsive.

[kmaforty]

Thanks all for the input. To be sure, this is not by design. I have logged 120k of the 220k miles that are on the car. The brakes have always been very good and is a big reason why the car is so fun to drive. This issue was first noticed about a year ago coming down a sloped parking lot and had to stop quick. It has gradually become more pronounced. I do not think it is related to any recent repairs, since I have not done anything with the brakes or intake manifold. I did replace the fluid (and it was overdue) hoping it would improve, but it did not improve.

To reiterate, When making a hard stop, there is very little brake force and the pedal is hard. Stopping distance increase. I've had a couple tense moments. I don't want to continue driving in this condition or I will "T-bone a Buick" at some point. During normal driving the brakes are OK.

juld0zer may be on to something. I may not have explained the test methods well, but the booster did not seem to be an issue. I will check the vacuum lines for leaks. I've read some threads, but there was some disagreement between members on the grommet leak. If anyone can point me to a good reference, that would be great.

[juld0zer]

The last two grommets I fitted had expired (yes, some parts have expiry dates). One was ordered from FCP and one from BMW in Australia. Both random overnight high and hard pedals were still not cured completely until I applied silicone sealant around the outer circumference. Both times I fitted a new check valve pipe also.
Sometimes the plastic firewall piece can interfere with the check valve installation angle too.

Is your car RHD or LHD? Do you mind posting a photo of your vacuum line layout if it is LHD?

The final possibility if all else checks out is a fault in the DSC module, which may not even detect a fault if other inputs have caused it to malfunction

[kmaforty]

Not sure if anyone has anyone has followed my recent posts, but I've set out to tackle a couple nagging issues on my 330i in the last month - oil leaks and a brake pedal/boost issue. (I also blew up my 550i motor, but that is another story). Not sure if the two 330 issues are related in anyway, but they both have gotten progressively worse. I created this thread looking for help with the brake issue and might be making progress but found more questions too:

Brake boost - I did find a couple small leaks in the vacuum line to the booster. I will purchase a new line and see if that helps my issue. Test method: I disconnected the check valve and connected a fitting to my compressor at low psi while I sprayed soapy water around suspect areas. You can see the leaks as evidenced by the bubbles in the first two pictures - the T and where it connects to the engine.

With the compressed air, I can hear a bigger leak from under the intake manifold towards the rear, possible the oil separator? I was not able to spray soapy water or identify where it could be coming from, but it sounded like a lot of air. That may explain why I could not pull a vacuum with my hand pump.

I tried to use a hand vacuum pump to see if it could hold 18", but could not pull any vacuum on the either direction of the vacuum line - either the line going to the engine or the side going to the booster - pic6. Should I have the brake applied when trying to check vacuum to the booster?

I did pressurize the line going to the booster with the compressor and can confirm there were no leaks in the line, at the grommet going into the booster, or on the booster itself - as evidenced by the soapy water. Or would the check valve prevented positive air pressure from entering the vacuum?

Juld0zer - sorry for the delay. Yes, it is LHD. and it is about impossible to access the grommet at the bottom of the booster under the maze of brake lines. Not sure if you can make it out in the pic 4. I think I will need to remove the master cylinder to access it if it is leaking. Any DIY's on that?

Oil leaks - I may have succeeded in eliminating the significant oil leak/s by replacing the valve cover and oil filter housing gaskets. I had responded to a DIY previously with multiple concerns during and after the job. I finally had to purchase a small torque wrench and re-tighten all of the valve cover bolts. They were definitely too loose when I initially torqued them by hand. I still managed to snap another (new) bolt in the re-tightening process, so I had to back down from indicated 7N-m + 90 deg to 6N-m + another 45deg. Once I clean up the underside of engine, I will know for sure, but there appears to be fewer spots on the garage floor and the burning oil smell has improved.

The underside of the car is still covered in oil all the way back to the second belly pan, pic 3. I am hoping the oil had just run down from the top and not the oil pan leaking too - but it is a lot of oil!. So to be sure, I pulled the belly pans to access the engine and clean it off. (I was also hoping to see if I could find the source of the vacuum leak from the underside as well, which I could not).

When I pulled the front pan, I found the remnants of some type of seal (maybe 2" OD) scattered about - pic 5 & 7. I have no idea what this could be, but thinking it could be the source of either the vacuum leak or another oil leak. There were at least a dozen pieces, some longer and possibly fiber reinforced, so it was not something that kicked up off the road. It had to come off my car.

Anybody have any idea what this mystery part might be?

[PerfectAce]

Looks a lot like shredded belt to to me. What's the condition of your serpentine belt? Has it failed recently or been replaced due to damage?

I'd be having a good long look at your front main seal, if I were you.

[juld0zer]

PerfectAce is spot on. I would avoid/minimise running that engine. Those are serpentine belt and front main seal parts. Your oil filter housing leak has soiled the serpentine belt, causing it to swell and weaken then shred. Usually the torn belt then wraps around behind the harmonic damper and pushes in the front main seal so I would also strongly suggest looking closer.

What PSI did you do the testing with? Vacuum systems should generally not be tested under positive pressure as damage may occur or false leaks may appear (vacuum actually sucks connections tighter, eg rubber hoses on hard plastic or metal barbed fittings). In our case, this general advice has to be disregarded aa these thermo bonded plastic lines as normally capable of holding light positive pressure. BMW uses them even on the latest B48 engine coolant lines. Vibration and age can cause them to leak at the joiners, as you have found.

The audible leak under the manifold could be the second bubbly photo you posted - the booster line to the vacuum pump (visible between the intake runners). Those two leaks you found would cause weaker brakes due to less servo assistance and a brake pedal that is harder to press than normal. However, with your test using compressed air, the check valve at the pump will allow air to enter the pump and crankcase so that air has to go somewhere - oil separator, intake manifold, crankcase..
You have to apply vacuum to this valve to check its operation.

There is a chance your fittings to the booster check valve line (the female end of the quick disconnect fitting) were not airtight enough and hence you were unable to generate any vacuum by hand. You have to use the right fittings to get a reliable result, either get a pipe from a wrecking yard or replace the lot with new pipes.

You could remove the small vac hose off that T piece and try vac it from there. That will test the engine vac pump check valve, the booster, its check valve and grommet at the same time but the leaks you found will impair the result.

Check valve grommet at the booster is almost as difficult to access on our RHD models. I usually use a long metal trim clip tool (the two prong fork type) or a long prybar to pop off the valve and grommet. Installing the new grommet is fiddly and may be more challenging on your car. You may have to use a coathanger and guide it into the hole and use the prybar to push it on. Use silicone spray or rubber care to lube it up.

Valve cover bolts are aluminium so should not be reused once stretched to spec once.

Could you get an assistant to stand on the brake pedal while you check how much the booster/master moves? Maybe this movement is cocking the anchored check valve pipe and opening up a gap between the check valve and worn grommet? You can do this test with the engine off. You can check again with the engine running after you confirm the front main seal is intact and no belt is wrapped around it

[kmaforty]

OIL leaks:
Good call on the serpentine belt PerfectAce and juld0zer. Belt is intact but appears to have some fraying at the edge. I have a new belt on order as well as front main seal ready to go if there appears any damage when I pull the damper. I just need to get my hands on the special seal puller/installer tool.

There appears to be some leaks persisting from below, so I will need to verify the source. I discovered there were a few oil pan bolts that had become loose and certainly adding to the issue. I have re-torqued in hopes that minimizes the leaks until Mr. Motivation arrives and decides to do a proper oil pan gasket replacement.

Vacuum Leak / Hard Brake Pedal:
I used only 5-8 psi tank pressure to test for leaks. I can understand your concern with positive pressure testing and causing false leaks or damage. The 2 leaks I found were probably minor and maybe a "false leak" at the engine fitting, but will at least try to replace the small tube at the T - it was looking pretty soft and ratty.

I did go back and connect my vacuum gauge/pump at the male-side of check valve disconnect (shown in the picture) and test for vacuum. I am pulling 28psi at idle, so maybe that is an indication the system is good up to the check valve?

The gauge /pump seemed to have an airtight seal to the pipe when I was testing. I really could not get ANY vacuum, but I will try the vacuum test again and make sure I have a good seal. I will also try the brake check for movement at the booster.

Getting at that grommet looks impossible. I am afraid of pulling out the old grommet and not being able to wiggle a new one into place. Is it possible to remove the reservoir and master cylinder to get access without draining the brake fluid?

[juld0zer]

I haven't seen a LHD one in person so I'll let someone with first hand experience share their tips.

On a RHD car you can undo the master cylinder and slide it out of the booster a bit but you are supposed to replace the oring between the master and booster. I'm not sure if you want to add another possible leak to your list at this point. I've honestly never replaced that oring and I've done several master and booster replacements. If fluid has spilt in the past and dribbled down the booster it may have rusted and can cause a leak here too.

[kmaforty]

Finally decided to have my indy look into the brake issue. He found the booster actually had a hole in it from corrosion - I did not see it, so not sure where. He also said the master cylinder was bad, so it was replaced as well. Brakes are now working as they should. thanks everyone for the help.