Brake Failure: Pedal Sinks - What to Do?

Hello everyone!

While I was waiting at a slightly uphill intersection yesterday, I had my foot on the brake pedal.

Suddenly, I heard a short hiss, and the brake pedal pushed itself outwards.
The braking effect was zero. As soon as you tried to press the pedal, you could feel it stiffen and push outwards.

Do you have any ideas?

I have a §57/a review today, hopefully I can pass it.

Best, Kylie

It might be interesting to know which classic car is being discussed.

Regardless, I wouldn't care about the appointment, or is it more important to you than a working brake? I hope you're not just fixing it because you have an appointment?

Best regards, Rainer.

"There seems to be a problem with the BKV (brake control unit). A valve may not be working correctly, or there might be a leak in the line. In any case, the vacuum is being lost (venting). Check to see if you can hear or feel where the air is leaking from."

Ow, sorry, it's a Skoda Octavia, model 1U, manufactured in 1997, with a 1.9 TDi engine.

Best,
Kylie

The workshop manager explained and showed me the day before yesterday that the vacuum was present, meaning that only the BKV (presumably a specific component) must be defective. After an extremely expensive replacement and a significant amount of labor, I received a call yesterday stating that the vacuum pump was also defective.
(allegedly, the BKV was destroyed due to a defective vacuum pump?!)
'...and that, even though he was still here the day before yesterday? The whole thing seems very suspicious to me.'

Can a faulty BKV (battery control unit) only be tested for functionality when it is installed?

Best, Kylie

kylie wrote:

The day before yesterday, the workshop manager explained and showed me that the vacuum was present, so it must only be the BKV (Boost Control Valve) that is defective. After an extremely expensive replacement and a lot of work time, I received a call yesterday that the vacuum pump was also defective. (allegedly, the BKV was destroyed due to a defective vacuum pump?!) ...and that, even though he was working here just the day before yesterday? The whole thing seems very suspicious to me.

It does to me too: as long as the vacuum pump doesn't suddenly produce a large amount of overpressure (which, in my opinion, is normally impossible), a vacuum chuck cannot be damaged due to a lack of vacuum.
And "too much vacuum" doesn't exist, because -1 bar would already be absolute 0 bar, and anything less is physically impossible.

Quote:

Can a defective BKV (battery control unit) only be tested for functionality when it is installed?

Unfortunately, I don't know.

No, it should be the other way around.
The BKV (presumably a component in a system) is leaking, which causes the V-pump to run continuously and eventually fail. Normally, the piston remains outside when the vacuum is at its maximum, and the NW (presumably a rotating part) spins freely without having any effect.
Then, perhaps the vacuum pump wasn't working correctly during the test beforehand?

Hmmm.. It seems that the failing BKV (battery control unit) is a typical problem for the A3, Golf 4, and Octavia platforms.
The same thing happened to me at 280,000 km.
Standing at the traffic light, suddenly the brake pedal became hard.
When you release the brake pedal, negative pressure builds up again.
When you press the brake pedal, it makes a hissing sound, and the pedal becomes stiff.
I then bought the BKV with the exact same part number.
However, in the future, I would choose the one with the "H" ending, as that is the strongest.