[Three_thirty_I]

Hi guys, so about two months ago I started to notice something strange with my car's brakes, after cruising for a while without touching the brakes, when you brake, the pedal goes in as normal and there's a brief moment where the pedals dips down more, as if it is losing brake pressure. I don't have the common hard brake pedal or the brake pedal that goes to the floor, but this is quite annoying and could eventually get worse. The car was at a workshop for other work but I did ask them to see if they could see anything wrong, suspected a leaking brake booster, but all they found was some fungal contamination in the brake fluid (it does get flushed every 2 years BTW) which I can only suspect comes from the fact that most workshops including BMW don't bother to flush the line to the clutch (manual). So they flushed and replaced all the brake fluid, but this issue persists. There are no fluid leaks, and the fluid level had not changed, so all I can think is that it could be a vacuum leak? I know that it is a common issue that the vacuum hose with the check valve and grommet start to leak, but that usually cause the hard brake pedal or brake pedal to the floor issue, but maybe this is related and just not as severe? With the engine off, if I pump the brake pedal it goes hard as expected, and then while hold the pedal and starting the engine it does quickly returns to its normal position, so that shows that the brake booster is working as normal, but if I press the brake pedal quite firmly, it's as if the pedal does feel like it keeps going down, but by a very very small amount, to a very minor extent, I could say that the brake pedal feels squishy, but very slightly.

Any ideas?

[Three_thirty_I]

Was kinda hoping someone here would have experienced this or had some advice. But I gather it may very well be related to the common check valve or especially the rubber grommet/gasket that tends to eventually fail. So I will investigate this when I have time to dig in. Also found this thread which I briefly scanned though, so will go through it properly first. Hopefully I can restore my brakes to their normal operation!

[lowrydr310]

It sounds like a minor vacuum leak. I'm not familiar with the South African 330i but I'm assuming you have a N52B30. First thing I would check is the brake booster vacuum line; the mostly hard plastic tube going from the quick connect near the booster to the vacuum pump. You need to remove the alternator to access the connection at the vacuum pump. I recently removed my intake manifold and while trying to remove the vacuum line where it connects to the pump, it snapped off. The rubber hose connected to the nipple on the vacuum pump was solid and brittle, probably expected after 13 years of being exposed to engine heat. Instead of buying a new one I used a piece of fuel line tubing to reconnect and it's been working well so far.

Then there's the other end of that hard tube, with the connector and the o-ring/grommet. If that all checks out, the next thing I'd look at is the brake booster itself. The diaphragm may have a small hole or tear.

[Three_thirty_I]

Originally Posted by lowrydr310

It sounds like a minor vacuum leak. I'm not familiar with the South African 330i but I'm assuming you have a N52B30. First thing I would check is the brake booster vacuum line; the mostly hard plastic tube going from the quick connect near the booster to the vacuum pump. You need to remove the alternator to access the connection at the vacuum pump. I recently removed my intake manifold and while trying to remove the vacuum line where it connects to the pump, it snapped off. The rubber hose connected to the nipple on the vacuum pump was solid and brittle, probably expected after 13 years of being exposed to engine heat. Instead of buying a new one I used a piece of fuel line tubing to reconnect and it's been working well so far.

Then there's the other end of that hard tube, with the connector and the o-ring/grommet. If that all checks out, the next thing I'd look at is the brake booster itself. The diaphragm may have a small hole or tear.

Thanks for the reply, yes, this has the N52B30 motor. I had the brake booster checked when my car went in for other work and all they found was some traces of fungal contamination in the brake despite it being changed every two years. But it is a bit intermittent, so when they tested it they probably didn't experience it for themselves. Yes, based on the age, it's most likely that some of the hoses and other rubber components are brittle or failing in some way, so will definitely inspect that entire line starting from the check valve end to the vacuum pump. But will only get time to investigate from next week sometime, so will see what I find.

[juld0zer]

Keep in mind the basics regarding a vacuum servo.
If you had a vacuum leak, the pedal would be harder to press and possibly travel less before the pedal firms.

I have experienced something similar to you but I could only reproduce it in city driving conditions. It was the DSC valve block. No faults ever logged. I suspect one of the valves was sticking. The pedal would have the same normal travel but as I neared the car in front and needed more brake, I press the pedal harder and the pedal would sink down another step. The step was noticeable and is the best way to describe it as it is a sudden drop down.

[Three_thirty_I]

Originally Posted by juld0zer

Keep in mind the basics regarding a vacuum servo.
If you had a vacuum leak, the pedal would be harder to press and possibly travel less before the pedal firms.

I have experienced something similar to you but I could only reproduce it in city driving conditions. It was the DSC valve block. No faults ever logged. I suspect one of the valves was sticking. The pedal would have the same normal travel but as I neared the car in front and needed more brake, I press the pedal harder and the pedal would sink down another step. The step was noticeable and is the best way to describe it as it is a sudden drop down.

I still need to investigate this. But yes, the very best way that I can describe the sensation is, it's as if the pedal is physically snagging on the initial biting point, then suddenly allows the pedal to depress further. So in a manner of speaking, for that brief moment I could say that the pedal feels harder than normal, but not excessive. It's just very annoying when it does it. I will definitely keep the DSC in mind, thanks!!

[juld0zer]

You're better off replacing the whole module unless you have no means to code the used controller to your car. Use a module that has an identical label to yours (the white label on the motor housing)

[Three_thirty_I]

Originally Posted by juld0zer

You're better off replacing the whole module unless you have no means to code the used controller to your car. Use a module that has an identical label to yours (the white label on the motor housing)

Hmm, sounds expensive! I will have to also ask the mech that had my car awhile back if he considered this and investigated. The problem is that I don't think he was able to replicate it very easily, and even now, it behaves most of the time, albeit, I can usually feel it subtly here and there. What on earth causes this issue? Well, from some searching, seems like the start of all of this appears to be a common cause, a sudden avoidance braking and all of a sudden the brakes aren't happy. For what it's worth, while it's a 2005 model, the mileage is crazy low for the car's age, so I cannot think that it's a normal wear and tear issue, maybe pure age, but certainly not from high mileage, fresh brake fluid every 2 years, and brakes aren't ever abused.

[Supermoon]

Replace brake vacuum hoses (3 total. 2 of them are easier to replace). Most likely the culprit is the one that attaches directly to the brake booster. It has a checkvalve that can leak.
Do replace the gasket between the hose and the brake booster too. It's known to cause vacuum leak.

[Three_thirty_I]

Originally Posted by Supermoon

Replace brake vacuum hoses (3 total. 2 of them are easier to replace). Most likely the culprit is the one that attaches directly to the brake booster. It has a checkvalve that can leak.
Do replace the gasket between the hose and the brake booster too. It's known to cause vacuum leak.

Yes, that will definitely be my starting point since those are probably suffering from age now. Hopefully it's just that. Thanks!