Mk4: (Audi A3): Racer build. Struggling to get the second brake circuit to bleed??

Race car, so I have removed the ABS unit and piped directly.
My original A3 master cylinder has 2 bleed nipples on it, one nearer the servo bleeding the first piston. The second piston at the pointy end of the master cylinder is bled using the other nipple.
My issue is no fluid coming out of the second nipple, even with the pedal right down.
Suspecting a jammed second piston I removed the original A3 master cylinder(non user access) circlip to examine the insides.
Nothing obvious.
So, went to the breakers and got another master cylinder (Golf mk4, exterior nearly identical)
Fitted that, bled the first circuit fine, solid pedal.
But still no fluid out of the front port bleed nipple, so no pressure to two of the wheels.
About to buy another master cylinder (no.3) , and start again.
Anybody had these symptoms?
Also, the mk4 Golf master cylinder seems to have less stroke than the Audi one, so maybe they aren't identical inside?
Been advised to bleed the master cylinder before bleeding each wheel, this seems to work well, but only on half the circuit?
Any help appreciated.
(Its an A3 8L 1.8T Sport model around 1999, probably had stability control, as OE master cylinder has 2 feedback switches)

 

If the cylinder has a different stroke that would suggest an issue. Can you reverse bleed it?

 

Here what you are saying, not sure how it would operate when correctly bled. With one circuit ( the 'good' one) bled there is a proper pedal, but only two wheels braked.
Therefore, does that take the 'stroke' question out of it? ie the pedal only moves an inch or so.
Other thoughts;
I tested the cylinder before fitting it (mk4 golf one) and filled the reservoir. Pumping the plunger spewed fluid out of both ports. (I was flushing..)
So the second port does pass fluid when off the car..
Also, I have bled these without the engine running, is that important, or not? (ie no servo action)
So, just back from collecting a third master cylinder, will try that also.
And compare the plunger strokes before fitting it, this may tell me something..
Thanks
Jon
Like most faults there is a learning opportunity here...for sure..

 

I always bleed servo'd brakes with the engine running.

 

Having tested all three on the bench, I now think the system is not true 'dual' circuit. It looks the primary circuit is the one nearest the servo (which was working correctly at the start of this thread) and the 'secondary' circuit (other 2 wheels as its piped in my case, now) only comes into operation
when there is a failure in the primary circuit. (ie at longer pedal strokes)
Its not conclusive from my rather poor bench tests, but that is the current thinking..
To get the master cylinder out, the engine mount and lots of other stuff has been taken out, so if I need to have vacuum to bleed properly, I will have to put this back together.
Anybody else confirm that its necessary to have the servo functioning?
Thanks
Jon

 

@Jon Olds , just thought, there was something on the A4 that would cause problems if fluid lost, but that may have been electronic as they are quit a sophisticated beast, think it didn't allow it to be bled after a major, quick loss of pressure, I'll try and find it later.
But there isn't a shuttle valve in the cylinder at all? Again had trouble on the mg when changed calipers and it was bled by foot, and so no resistance to start.
Does seem odd having multiple fail on you, haven't got crud anywhere blocking a line?

 

It was bought as a project, so a collection of bits really. The master cylinder has been off before, so it may not be the original.
I stripped the first down, but not sure if I would recognise a shuttle valve if I saw one.
Started bleeding using a pressure bleeder, then by foot, seems whatever I do there isnt a significant flow from the front port or bleed nipple.
Not sure the ones I have are 'failed', more likely I am not understanding the original designers intent.
Still looks more like a master slave system rather than a symmetrical dual system.
Moving onto the next problem to allow some thinking time on this one
Thanks
Jon

 

Is it split front and rear, or diagonally, from the cylinder?

 

Beat me to it by 14 minutes, pah.

 

Master cylinder not marked, ABS unit not marked. VW have used M12 and M10 fittings sizes to differential left and right channels
on the ABS pump.
Haynes manual and original plumbing align to being split diagonally, logically.
Haynes manual contradicts itself by mentioning a 'dual' circuit implying its fully symmetrical then referring to the Primary and Secondary circuits....
Yes, I know I will have to deal with excessive pressure leading to over aggressive rears.
Regards
Jon

 

Interface into the plunger slightly different lengths on all 3 cylinders, but not so much that it accounts for less travel on the golf mk4 one, when installed.
Also, the pushrod isn't fouling on the (in)side of the tube.
Think refitting the A3 one and trying to bleed the 'secondary' (outer) plunger first could be the next move.
Or taking the mk4 golf one to bits too, to have a look inside...
Could convert the system to single circuit, that would definitely work, but think there's more to learn here first
Jon

 

https://www.audi-sport.net/xf/threads/brake-problem-audi-dealer-cant-bleed-them-even.124741/
Don't know if any info in that Jon, and the shuttle valve was in master cylinders on minis, split circuit pre servo, they used to close off one outlet if pressure drops and activate the switch for warning light, used to corrode and stick like toffee.
Some forums have mentioned about vagcom being needed to bleed the A3 system, not sure if you've removed anything?

 

Thanks
Jon

 

Thanks guys,
Yes, Jonnie, my first thought was to raid the stock of old master cylinders.
But, the mounting bolts aren't horizontal.
One I bought yesterday (mk4 golf) has the mounting bolts symmetrically opposite the other two I have.
so that'll be 'in stock' now (it was cheap, so who cares)
That'll teach me
LOL
Jon
So, I'm back to trying to bleed the cylinder 1 and try the front 'secondary' port first.

 

Just read this tread... A bit off topic... As the title `` Racer build ``...im no fan of ABS in a racecar ( even on a roadcar ) .... imo its not needed... as long the system is properly in balance and the driver has enough pedal feel to control things...

Up till 2007 i had an A3 as a roadcar ...
In normal use....it worked ( took action) only a few times over the years....so far so good


Until you cross the line....when you drive it beyond what it is programmed for.. like motorsport use
brake very hard and late on an uphill road, ( like exit motorway ) wheels fully in contact with road and no sign of lockup ( and with proper tyres )
Until ABS decided that this was to much slowing down under braking and it could not be wright .... and it should take action.... so it took brake pressure away .... Pretty scary when you as a driver becomes passenger...
I pumped the pedal myself a number times and got the car stopped in time.... but it learned me a lesson

 

HPR; Agree, my first rally in my Pulsar, ABS no use at all with big AP brakes/calipers fitted. Dash on,off [power..]
Has never been turned on since….
Jon

 

Still no pedal, LOL.
Decided to go single circuit with a GTIR (nissan) pressure limiter in the back brake circuit(s).
Made a blank for the front circuit (M12 fitting with a brass dummy pipe with 45 degree taper)
So, refitted the original master cylinder, no joy.
Refitted the second unit and now cannot get any fluid out of the 1st chamber , exactly opposite the symptoms I had first time.
Makes no sense to me.
Tried vacuum bleeding, foot pedal bleeding, tried pedal up/down.
Haven't had the engine running, but can't see what difference that would make.
Tommorrow, planning to buy another used master cylinder.
No had any significant problems bleeding VW brakes before, so am struggling to understand why these are different.
The 'system' is only a series of T pieces now, with the pressure limiter in the rear circuit
Jon

 

Success.
The hottest of hot tips I can give you guys after hours and hours of dicking around;
Get a 60ml syringe, 1/4" plastic tube, resistant to brake fluid
Clean container 1/2L of new fluid in
Syringe the system as full as possible directly into the two bleed nipples on the side of the master cylinder
Open caliper bleeds in sequence.
Keep master cylinder topped to top
Bleed as normal
Worked for me, anyway, LOL
Why? (I'm not exactly 100% sure..)
Jon