Brake pedal very soft, tried all the usual things

[sdutton007]

I've had an issue with my brakes since I had to replace the rear calipers - even with the engine off, the pedal is very soft. The first time you press it, it goes to the floor with minimal resistance. After that, it's a bit better but still really soft but if you wait a few seconds it will again go to the floor with almost no resistance.

I've done the following in an attempt to fix this:
- bled the system (about 20 times) using manual, gravity and pressure bleeding
- replaced the rear calipers with ones from a working A3
- replaced the master brake cylinder twice (one off a working A3, one brand new from Audi)
- bled the ABS pump via VCDS
- replaced the ABS pump

I've also left the system pressurised for hours and checked for leaks along the brake lines and calipers.

I can even clamp all 4 brake pipes and the pedal is still soft.

Also, the brake lights now stay on permanently (with ignition on) but I suspect this is probably the brake light switch due to having pumped the brakes so many times.

Does anyone have any other ideas what it could be?

 

[NHN]

What are you using to bleed?

The gunson unit is very good, made world of difference to when I was working on my abs upgrade module swap.

 

[e9_mustii]

What are you using to bleed?

The gunson unit is very good, made world of difference to when I was working on my abs upgrade module swap.

Abs module upgrade ? Tell me more Nige
P.s you forgot my Quote again

Assuming youve check the soft brake pipes for splits/leaks and you not losing brake fluid ?

Sent from my SM-N976B using Tapatalk

 

[NHN]

Ah man, so sorry, I'm besieged & parts availability is seriously compromising for installs tbh, it's causing mega issues for quotes amongst others.

Can you remind me again please mate.

Abs upgrade was to get hill hold, I've got to upgrade it's again at some point to get other options, but it's more of a want than need as need to change cars at some point.

 

[sdutton007]

What are you using to bleed?

The gunson unit is very good, made world of difference to when I was working on my abs upgrade module swap.

I'm using a Sealey vs820 pressure bleeder but have also tried the 2-person manual method and gravity bleeding. Nothing has helped.

Abs module upgrade ? Tell me more Nige
P.s you forgot my Quote again

Assuming youve check the soft brake pipes for splits/leaks and you not losing brake fluid ?

Sent from my SM-N976B using Tapatalk

Yep, there's no leaks and it's losing no brake fluid. I can also leave it pressurised for hours and not lose any pressure.

 

[sdutton007]

Is there any way to test the master cylinder? Despite replacing it twice including a new £300 one from Audi, the symptoms seem to point to that.

I was thinking maybe I could either loop it or just block off both connections?

 

[ahtcap]

Read somewhere,you may get an air pocket when changing calipers.
Try moving the caliper,then bleed the brakes with the bleed nipples at the very top.

 

[sdutton007]

Read somewhere,you may get an air pocket when changing calipers.
Try moving the caliper,then bleed the brakes with the bleed nipples at the very top.

I can try it, but the issue still happens if I clamp off all 4 brake lines.

This problem makes no sense, all I know is that Audi brake systems are really badly designed and have serious design flaws.

 

[NHN]

It's an air pocket imho.

 

[sdutton007]

It's an air pocket imho.

Where could the air pocket be? I've bled the entire system including ABS pump and clutch, clamped all brake pipes and the master cylinder doesn't have bleed nipples (I know some do but neither of my A3s do)

 

[ahtcap]

Where could the air pocket be? I've bled the entire system including ABS pump and clutch, clamped all brake pipes and the master cylinder doesn't have bleed nipples (I know some do but neither of my A3s do)

Did you manage to get to the bottom of this?

 

[sdutton007]

Did you manage to get to the bottom of this?

Nope It's been off the road for over 18 months now, I can't figure it out. I even got a good mobile mechanic out and he couldn't figure it out either. And I don't have £2000 spare to get an Audi specialist to fix it.

Aside from ripping out the entire brake system and replacing it with a properly-made brake system (i.e. nothing made by Audi/VW), I really don't have a clue what to do!

Plenty of people have had the exact same problem, but nobody seems to have a clue how to fix it

Are you having the same issue?

 

[ahtcap]

Nope It's been off the road for over 18 months now, I can't figure it out. I even got a good mobile mechanic out and he couldn't figure it out either. And I don't have £2000 spare to get an Audi specialist to fix it.

Aside from ripping out the entire brake system and replacing it with a properly-made brake system (i.e. nothing made by Audi/VW), I really don't have a clue what to do!

Plenty of people have had the exact same problem, but nobody seems to have a clue how to fix it

Are you having the same issue?

Just bought my son a 61 golf which has a firm then spongy brake pedal.
I've done a full brake fluid change and its still the same .

 

[sdutton007]

Just bought my son a 61 golf which has a firm then spongy brake pedal.
I've done a full brake fluid change and its still the same .

I'd say it's most likely to be the master cylinder - the Audi/VW brake master cylinders have 2 major design flaws:
1) If the brakes are bled the "normal" way, the pedal moves further than normal and destroys the internal seal.
2) If the brake piston is rewound without taking the cap off the master cylinder, it can flip the internal seal.

This part is £300 at VW/audi, or you can risk a 2nd hand one.

But as you can tell by the this thread, there's no guarantee that this will fix it.

The solution I use is just to drive round in a 25 year old Toyota - it may not be as nice as the Audi, but it never goes wrong!

 

[kuebk]

Had similar issue, after replacing both rear calipers and bleeding with 2-person method the pedal was softer than it was before. What I did was to bled the brakes again, 250ml of brake fluid per caliper. There was no air coming from the caliper only the fluid, dunno how but it helped somehow.

How much of brake fluid did you pour per caliper?

 

[sdutton007]

Had similar issue, after replacing both rear calipers and bleeding with 2-person method the pedal was softer than it was before. What I did was to bled the brakes again, 250ml of brake fluid per caliper. There was no air coming from the caliper only the fluid, dunno how but it helped somehow.

How much of brake fluid did you pour per caliper?

I made sure I drained at least 250ml per caliper and have done it at least a dozen times now