Brake Problem: Faulty Vacuum Pump?

Hello,
I'm having a 'small' problem with my brakes:
Here's the translation:

'To start with: The brake booster seems to be okay. The pedal is hard when starting the car, but then it softens about a third of the way down. There's no air in the system, so everything seems fine. Normal braking also works well. However, if you brake repeatedly in quick succession, the effect of the brake booster seems to diminish, and the pedal becomes hard. So, if you press the brake pedal hard three times in a row, the brake assist is almost completely gone on the third braking. Because you're shifting gears during this, you're probably pressing the clutch, meaning the engine is idling by the third braking. Could it be that the vacuum pump isn't able to maintain the pressure when the engine is idling? As I said, the initial braking is very powerful, as you'd expect from a G4. So, the system itself seems to be in order.'
Has anyone ever experienced something like this before, or does anyone have any ideas?
Regards,
Jens.




Golf 4 Sedan, 220,000 km, 90 hp, AGR.

Hello,
I suspect a reduced pumping capacity of the vacuum pump, possibly in conjunction with a small leak.
Best regards,
Günther.

My system behaves the same way. If you provoke it with pumping the brakes, the assistance stops after 3-4 times. Therefore, I can only warn against performing full braking maneuvers from high speeds while the car is in a neutral state. If you have to brake in multiple stages due to traffic conditions, the assistance will suddenly disappear.

That's Diesel...

"It's the same with mine. If you brake a few times in a row while idling, it goes back to feeling like the old Beetle days (no more power assist). You might want to check the hard plastic hose from the brake booster for cracks (it certainly won't hurt to do so). Mine was quite torn in several places, especially at the connection points. However, in my case, the power assist was gone after just one brake application."

My system behaves the same way. When you provoke it with pumping, the assistance stops after 3-4 times of braking. Therefore, I can only warn against making full stops from high speed while idling. If you have to brake in several stages due to traffic conditions, the assistance will suddenly disappear.

Das ist Diesel...

Hello,
Then, it would be advisable to disengage the clutch late during braking.
I'm going to try it out to see if I have the same behavior with the ARL.
I find it somewhat 'reprehensible' that the vacuum pump is so weak.
Best regards,
Günther.

Hello everyone,
Thank you for your interest.
I probably should mention that I've only recently noticed this behavior. Shouldn't I have noticed something like this after 200,000 km? I'll definitely look into the tip about the hoses in the area of the connections. At first glance, I didn't see anything, but maybe there's a small crack that's hard to see.
See you!
Jens.

Hello everyone,
Thank you for your interest.
I probably should mention that I've only recently noticed this behavior. Shouldn't I have noticed something like this after 200,000 km? I'll definitely look into the tip about the hoses in the area of the connections. At first glance, I didn't see anything, but maybe there's a small crack that's hard to see.
See you!
Jens

Hello,
'With my ARL engine, I didn't experience this effect today! I disengaged the clutch and braked hard multiple times, and there were no unusual reactions!'
During the first inspection of my AJM engine, a hose on the brake booster was replaced due to 'heat damage.'
Best regards,
Günther.

Günther,

Did you brake hard repeatedly, releasing the pedal completely in between?

"Especially, avoid releasing the brakes or accelerating while in motion. I noticed this when maneuvering, where I was rocking back and forth in idle and pressing the brakes a few times. That's when the assistance stopped working."

Günther,

Did you brake very hard multiple times in a row, and release the pedal completely in between?

Hello Roger,
Yes, but only at a speed of 80 km/h, as I was only driving on a country road.
Soon, I'll try it out on the A3 highway when traffic allows, at 180 km/h (it's not possible to go any faster yet due to the weather conditions).
Best regards,
Günther.

Hi everyone,

That's not normal!

I also tried it today, and I couldn't observe anything like that, not even when I was stationary with the clutch disengaged and pumping the brake pedal.

I think something like that would have been noticed a long time ago, given my driving habits, because in the mornings when I drive to work, I rarely exceed 1200 RPM in city traffic, and in traffic jams and when maneuvering, I'm usually in neutral, which also involves a lot of braking.

@Guenther: I don't think you'll notice any difference other than what you'd experience when pumping while stationary, even if you disconnect the coupling at 180.

Best regards,

Jan.

Hello Jan,
I also can't imagine that this is 'normal'; it would be a 'significant' security vulnerability.
Best regards,
Günther.

@Jan and Günther:

I'm not talking about gentle braking like when maneuvering, but rather very strong braking where the BAS (Brake Assist System) kicks in. After just a few instances, it literally loses its effectiveness.
Not only with my car, but with EVERY diesel vehicle I've driven so far from VW as a rental car.

Hello Rager,
'Sorry, can this be done multiple times without having to accelerate 'properly' again?'
I think the brake fluid return system might be 'overwhelmed' in this situation. To be honest, I don't have any idea why! The brake booster essentially just applies force to the brake pedal, and that shouldn't create 'excessive' vacuum in the system, should it?
Best regards,
Günther.

The BAS completely releases pressure in one go, which causes the pedal to drop significantly once a certain threshold is exceeded.

Hello Roger,
'Then it would also indicate a car. And who would do something like that repeatedly? In that case, the brakes should also fail relatively quickly due to vacuum loss, even when going downhill in neutral. I know that you're not supposed to do that!'
Best regards,
Günther.