Audi A4 B6 - Brake warning light on first thing

Honest John Back Room » Technical matters
View Threaded
Fri 11 Nov 2016 17:51
Audi A4 B6 - Brake warning light on first thing - Steveieb
The brake warning light comes on first thing but goes out when I press the pedal
The pads are fine but when I tried to fill the reservoir I was unable to get any fluid past the filter in the neck of the filler.
Tried bleeding the reservoir with the bleed screw to no avail. The level is above the joint but not quite up to the max.
Could this trigger the brake alert if there is a sensor in the top of the reservoir and is it possible to remove the filter?
Reply | Report as offensive | Link
Fri 11 Nov 2016 20:10
Audi A4 B6 - Brake warning light on first thing - Railroad.
The brake warning light comes on first thing but goes out when I press the pedal

That's what should happen. It's completely normal.

Reply | Report as offensive | Link
Sat 12 Nov 2016 10:15
Audi A4 B6 - Brake warning light on first thing - Steveieb
Thanks Railroad but this light is the large red warning light that fills the centre of the display as opposed to the small one which form the cluster at the top of the display
Reply | Report as offensive | Link
Sat 12 Nov 2016 12:16
Audi A4 B6 - Brake warning light on first thing - elekie&a/c doctor

Fluid level just below max will not trigger the brake warning light.There is no level sensor at the top of the reservoir.

Reply | Report as offensive | Link
Sat 12 Nov 2016 14:02
Audi A4 B6 - Brake warning light on first thing - madf

How old is the battery? If it is old and voltage low, you may get spurios signals. OK when alternator running.

Reply | Report as offensive | Link
Sat 12 Nov 2016 14:04
Audi A4 B6 - Brake warning light on first thing - pd

Quite common fault.

It is often that the battery is getting weak - a voltage drop causes loss of signal from the sensor which triggers the warning. Tends to happen when starting in the cold.

Reply | Report as offensive | Link
Sat 12 Nov 2016 14:09
Audi A4 B6 - Brake warning light on first thing - bathtub tom

I'm curious to the OP's Tried bleeding the reservoir with the bleed screw to no avail.

I've never seen a bleed screw on a reservoir. Have I led a sheltered life?

Reply | Report as offensive | Link
Sat 12 Nov 2016 15:49
Audi A4 B6 - Brake warning light on first thing - Steveieb
Thanks for all your posts.
I'm sure you are right about the battery getting weak and I'll get it replaced.
The bleed screw I refer to is in the top of the reservoir coloured yellow. It's quite large and if the filter is blocked on the main filling point which does not allow me to top up the reservoir , can I use this screw hole for topping up. Currently I cannot get any fluid to drain into the reservoir.
Once again many thanks for your knowledge.
Reply | Report as offensive | Link
Sun 13 Nov 2016 12:16
Audi A4 B6 - Brake warning light on first thing - Big John

Isn't the big yellow thing on top of the hydraulic fluid resevoir the top up cap (usually has level sensor built in)?

Reply | Report as offensive | Link
Sun 13 Nov 2016 18:07
Audi A4 B6 - Brake warning light on first thing - Steveieb
Thanks John. But it's not the. Screw cap which you undo to top up the fluid , there is a yellow screw in removable plug which can be removed with a screwdriver.
For some reason I can't get any fluid to drain through the filter in the neck of the reservoir and can only presume it is blocked
Reply | Report as offensive | Link
Sun 13 Nov 2016 18:40
Audi A4 B6 - Brake warning light on first thing - elekie&a/c doctor

I,m confused.There is only one way to top up the brake master cylinder with fluid,and that is via the large removeable screw cap.No need to bleed the reservoir,fluid is gravity fed to the brake (and clutch if fitted)master cylinder.If you are unable to get any more fluid in,it is probably full up.I doubt if this is the problem with the brake light coming on.I would suspect the fault is possibly battery related or pad wear indicator fault.Have you checked the rear brakes?

Reply | Report as offensive | Link
Sun 13 Nov 2016 18:45
Audi A4 B6 - Brake warning light on first thing - Steveieb
Thanks Eleciedoc. I am planning to test the battery. And I have posted a request on this site asking if it can be done in situ without damaging the ECU
Reply | Report as offensive | Link
Sun 20 Nov 2016 19:55
Audi A4 B6 - Brake warning light on first thing - Steveieb
Thanks Elecie doc and everyone else who recommended I changed the battery. Got it tested and yes it needed replacing.
Brake light fault cured and also got the drain hole cleared at the same time. Result!
Reply | Report as offensive | Link
Tue 29 Dec 2020 11:35
Audi A4 B6 - Brake warning light on first thing - Steveieb

Deja vu. Four years later !

The brake warning light is coming on even when the alternator is running and comes on intermittently over a journey.

Both the large red warning and the small one . So I guess the front pads may need changing?

New pads and discs on the rear recently.

Grateful your views !

Reply | Report as offensive | Link
Tue 29 Dec 2020 13:00
Audi A4 B6 - Brake warning light on first thing - elekie&a/c doctor
Could be front pads worn . May want to check the pad sensor wiring for breaks or rub outs . Runs down the suspension strut . (Can’t remember which side ) it’s incorporated with the abs sensor wiring.
Reply | Report as offensive | Link