Master brake cylinder

johnyfartbox

A2OC Donor
Had a poke around but cant really find anything.
Graham (Audifan) says its a pain to repair one, I think the MC is FUBAR...........after replacing the slave on the rear nearside I just cannot get fresh air free fluid through.
I have checked the feed pipe and it seems to be ok, no weeping. when turning the bleed screw im making sure ive not turned it to much that would allow air in through the threads.
I might have made the mistake of pressing the brake pedal to far down when pumping the breaks, i hope not because I have since read and been told by Graham that this should not be done, the MC overruns its normal max travel and the seals can be easily damaged.

Any feed back would be welcome.
All the best.
John
 

spike

Well-Known Member
have you tried bleeding the other 3 brakes. If they bleed normally, that would indicate the MC may be ok. Could be something as silly as the new bleed nipple is blocked / not fully drilled through

Cheers Spike
 

johnyfartbox

A2OC Donor
Thanks Spike, I thought for a while no one would answer.
I've not yet done the others, been busy looking after my youngest grand daughter but later I will try your suggestion.
Thank you.
 

audifan

A2OC Donor
Have you tried any other bleed point yet just to see if there is a flow of fluid? Did you let the reservoir get very low?
 

johnyfartbox

A2OC Donor
Hi Graham, yes, I cracked the bleed nipple on the off side slave and fluid dipped out of that one.
Don't know if there was any air in it though, used my mates self bleed tube and its opaque.
 

audifan

A2OC Donor
Do you still have the old cylinders? If so then take a bleed nipple out of one of the old ones and blow through it to make sure it is clear and replace the nipple in the near side slave cylinder. As you take the nipple out fluid should escape and some air will enter. Did you get the correct rear cylinders there are 2 types. Aluminium ones that are NOT for your car and cast ones which are. If you watch the fluid as it comes into a clear container you normally get an obvious colour change as the dirty old fluid is replaced at that point it is safe to say the air is out. My press 5 times then press 5 more normally does the trick but no harm in another 5 to be sure. DOT4 not all that expensive but only use DOT4 brake fluid.
 

johnyfartbox

A2OC Donor
I don't I'm afraid, fell to bits.
The slaves are the same because when I went for one they were a good price so got two so I would already have one in the garage, that's the one I have put on over the weekend.
They are both cast.
 

audifan

A2OC Donor
So you still have a spare one? If so either swap the nipples or replace the whole slave cylinder. Did you run very low on fluid when trying to bleed? Also see if both fronts bleed.
 

audifan

A2OC Donor
Remember the braking system is diagonally split in the A2 so rear left and front right are one one circuit and rear right and front left on the other circuit. Try starting the car and pressing the brake pedal twice, the cars has EBPD Electronic Brake Pressure Distribution. I would not suggest to bleed the brakes with the engine running.
 

johnyfartbox

A2OC Donor
I no longer have a spare, I used it on the side I'm having problems with.
That feed pipe let out quite a lot of fluid, I'm thinking now that I should have put some tape over the end but hey ho.
Ive not tried to bleed the fronts yet.

When this kit arrives, I will be tempted to flush out the whole system.
Thanks for the input up to now.
 

audifan

A2OC Donor
You could have screwed the old cylinder back onto the pipe and tie wrapped the pistons in. Not 100% on how the EBPD works. There MAY be a chance that so much fluid came out the system has shut down that line to prevent total fluid loss. More likely that line has drained the ABS pump causing it to shut off. If that is the case then the ABS pump will need priming and bleeding using VCDS. Have you taken the bleed nipple out and verified there is indeed a hole drilled into it? I most certainly would do a complete flush, do not forget the clutch slave cylinder also.
 

johnyfartbox

A2OC Donor
I found the old slave cylinder that I did a few weeks ago and swapped the bleed screw,
I've read about the ABS being exorcised by VCDS but I only have the lite versionot if I did the 75 quid registration would it then do the ABS?
 

audifan

A2OC Donor
Has the nipple change made any difference?
Do not use lite, but you have nothing much to loose. Think its ABS then output tests, someone may have to confirm this or give the correct location.
Also this from the manual


Special feature of vehicles with ABS/EDL


If a chamber of the brake fluid reservoir is completely empty (e.g. in the event of leakage in the brake system) the hydraulic unit must be bled via the "Basic setting" function and the fault reader V.A.G 1551.

And you could try this method...


Bleeding brake system

Bleeding brake system without brake filling and bleeding unit

  • Engine running
  • ‒ Build up pressure in braking system by pumping pedal
  • ‒ Open bleeder screw with hose of bleeder bottle attached.
  • ‒ Close bleeder screws with pedal depressed.
  • ‒ Repeat procedure until no further air emerges.
Let us know if any helped.

EDIT Looks like output test are not supported in lite free, do not know about registered version.
 
Last edited:

audifan

A2OC Donor
If you use a led torch and hold it against the opaque tube you can see the fluid and air as it passes by. The clearer the tube the better and white not good at all.
 
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