Brake problem - Audi dealer can't bleed them even

[TDIQ]

After a rear brake pipe split I replaced it which I thought would cure my no brakes problem. I then bled them. The brakes worked ok-ish but only towards the bottom of the pedal.

I took it to Audi who tried to bleed them and they couldn't, they said they'd done the master cylinder and the VAG-COM ABS bleed.

I then bought a pressure bleeder and bled them, which made them better but still the braking is at the bottom of the pedal. If you pump them they get harder like there's air in the system, but I bled 500ml out of each calliper and it was completely clear.

Audi suggested it could be a vacuum leak from a split pipe to the servo, could it be? The car has a rough idle when cold, and goes between 800 and 850/900 rpm every couple of seconds for about a second.

Any ideas please, this is driving me mad!

 

[s3dave]

Sounds like the master cylinder has flipped the seals...

 

[MUSH]

Sounds like the master cylinder has flipped the seals...

It does indeed!

When you bled the brakes did you take the pedal all the way down? When this happens it puts the seals into an unused position of the piston within the master cylinder so the seals flip over and its ********.

New MC id say

 

[a8 tech back]

run the engine

clamp all the flex hoses so the pedal goes rock hard

press and hold the brakes and get a second person to release the clamps one by one

if the pedal drops consistently to the floor replace the master

if it drops suddenly whilst removing the clamps re bleed that circuit

a3 can be a ****** to bleed after replacing the master but a tip

if the pads and discs are worn then this can also be a factor for spongy pedal

if you use vcds basic in address 03 follow the process completely

i find bleeding with a second person pumping till hard then open the nipple on the furthest calliper first and allowing the fluid to be forced out on the compression of the pedal more affective than the pressure bleeder set to 1 bar

do the same on the next calliper furthest away and end on the nearest calliper

the problem using the pressure bleeder is if it set to high

In the event of a filling pressure greater than 1 bar, a hydraulically actuated switching valve for the EDL control system prevents full flushing of the areas of the hydraulic unit specific to EDL. Proper bleeding is then no longer guaranteed.

or
Bleeding sequence

• 1 - Front left brake caliper
• 2 - Front right brake caliper
• 3 - Rear left brake caliper
• 4 - Rear right brake caliper

• [SIZE=-1]‒ [/SIZE]With bleeder bottle hose attached, leave bleed screw open until the brake fluid which emerges is clear and free from bubbles.
• [SIZE=-1]‒ [/SIZE]Press brake pedal 5 times to assist bleeding process.
• [SIZE=-1]‒ [/SIZE]Tighten bleed screw to 10 Nm, detach bleed hose and attach protective cap to bleed screw.
• [SIZE=-1]‒ [/SIZE]Repeat bleeding procedure in stated sequence at other brake calipers.
• [SIZE=-1]‒ [/SIZE]Making allowance for pad wear, fill brake fluid reservoir as far as max. mark and screw on cap.
• [SIZE=-1]‒ [/SIZE]Start engine and check both brake pedal travel and pressure.
• [SIZE=-1]‒ [/SIZE]In the event of excessive pedal travel, check brake system for leaks/repeat bleeding procedure

Press brake pedal slowly to avoid formation of bubbles.

• [SIZE=-1]‒ [/SIZE]Once pressure has built up, keep brake pedal depressed.
• [SIZE=-1]‒ [/SIZE]Open bleed screw with hose of bleeder bottle attached until pressure has been dissipated (pedal sags).
• [SIZE=-1]‒ [/SIZE]Keep pedal depressed and close bleed screw.

• [SIZE=-1]‒ [/SIZE]Release brake pedal and wait approx. 2 seconds to allow brake fluid to flow in from brake fluid reservoir.
• [SIZE=-1]‒ [/SIZE]Repeat procedure until brake fluid emerging is clear and free from bubbles.
• [SIZE=-1]‒ [/SIZE]Tighten bleed screw to 10 Nm, detach bleed hose and attach protective cap to bleed screw.
• [SIZE=-1]‒ [/SIZE]Repeat bleeding procedure in stated sequence at other brake calipers

there are also two bleed nipples on the master cylinder which need bleeding if the reservoir has run empty or you replace it that must be bleed prior to the rest of the circuit

 

[liquids3]

True!,or take the rear calipers of,put block of wood between the piston n caliper and try bleedin them upside down!,just this worked on my mates 200sx almost as if it had a pressure block of air trapped!,only worth ago!,because his was a pain!but mayby master!

 

[TDIQ]

Thanks for all the info, looks like I'll have to try a new master cylinder. Does anyone know how tricky it is to change and what's involved? Coolant bottle moved out the way and just change them over and bleed the system?

 

[TDIQ]

Still trying to figure out the issue. The pedal can be slow to return back up after it's used.

I got a good 500ml out of each calliper when bleeding so wouldn't it suggest the MC is fine?
What are symptoms of master cylinder failure?

 

[666markyboy]

A3 brakes are a flipping nightmare to bleed.... i tried allsorts on mine and came to the conclusion the master cylinder was at fault so changed it out for a new one and it went worse lol i even built a rig using a vacume cleaner to pull fluiid through rather than push it and it's still pants although they passed an MOT..... iv'e never got to the bottom of the problem allthough they have improved with time... still a bad pedal though compared to any other car i drive but the effeiciency is there,, just a very soft pedal

 

[666markyboy]

Ohh and iff you had a vacume leak then the pedal would go harder not softer

 

[TDIQ]

666markyboy: Thanks for the reply. I read your threads with your brake issues to try and figure out how to get to the bottom of this. It's a nightmare.

Are you 100% sure that the pedal would be harder with a vac leak? It's really strange because when the car is off, it's quite hard, then when I turn the car on with my foot on the pedal it sinks like it used to do before all these problems started.

I pump the pedal and it gets harder for a second or two then if I push again it's back to soft. All the braking is at the very bottom of the pedal and it's takes maybe a second to come from the bottom back up again. Definitely slower than before.

I don't want to start changing vac lines and the master cylinder if I can help it obviously, but surely it can't still be an airlock? I'm yet to try the VAG-COM bleed myself, will give it a go in the next couple of days.

 

[666markyboy]

Yeah the pedal goes hard when the engine is off because your getting no assistance from the servo.... you start the engine and the pedal goes down as the servo kicks in.... if you had a vac leak then the servo would not be working properly so the pedal would stay hard... the other thing i messed with which helped was stripping down the rear brakes as i though the self adjusters for the handbrake were a bit seized meaning the pads wern't being held as close to the rear disc as they could be..... someone mentioned clamping hoses earlier which is something i never tried... maybe if you clamp the rear hoses and see how that effects the pedal.... if it improves things that could also indicate a rear adjuster problem.... they do seize up over time... trouble is it's tricky to sort them as they inside the rear piston and not really classed as a servicable item.... i think Brakes International used to sell a service kit though.... i'll go have a look for ya

 

[666markyboy]

if you pump the pedal and it goes hard and you keep your foot on the brake does it stay hard or does it creep back to the floor ??? if it creeps back to the floor thats an indication of a master clylinder leak....

 

[TDIQ]

I've just had a look and if I pump the pedal so it goes hard then after a second it will start to sink down. I take it that it's the MC based on that?

 

[Y83OLK]

If you have trapped air it will do that also.

 

[TDIQ]

Might have to just keep bleeding it with the Gunson's pressure bleeder, and bleed the MC along with the VAG-COM ABS bleed as well as what a8 tech suggested to me then this weekend.

 

[TDIQ]

Is it possible to do the VAG-COM ABS bleeding procedure with just the front wheels off the ground does anyone know?

 

[jojo]

Is it possible to do the VAG-COM ABS bleeding procedure with just the front wheels off the ground does anyone know?

Why does any wheels need to be off the ground? The vagcom just activates the ABS unit when you are going through the bleeding process.

 

[TDIQ]

According to Ross-Tech it needs to be jacked up
Ross-Tech: VAG-COM: ABS Brake Pump Bleeding

Not sure why though, but don't want to ruin the ABS pump

 

[TDIQ]

Problem fixed!

Finally managed to sort it out today. First did the VAG-COM ABS bleed twice (with all 4 wheels on the floor). Don't know if that made any difference but then bled the master cylinder and pedal was rock solid. Brakes are better than ever.

Thanks for all the advice

 

[John ler]

An old thread I no, can I ask did you have any hissing on the pedel with this problem , ? I have same problem thanks

 

[wesley]

Thread revival! I also have exactly the same problem but also have a hiss at the pedal! Any help appreciated.