One caliper doesn't bleed manually but does with pressure bleeder

[sdutton007]

19 months on and still no working brakes! I haven't been able to get a hard brake pedal since I had to replace the rear calipers. Since then, I've re-replaced them, changed the master cylinder TWICE, changed the abs pump, bled everything over 100 times using the 2-person method, pressure bleeding and gravity bleeding.

This weekend, I noticed that on the front left caliper, either no fluid or a tiny dribble comes out when you bleed it using the 2-man method but it bleeds fine using a pressure bleeder. Now I'm thinking there's some weird problem with this caliper that might have been the issue all along.

Any ideas what could cause a caliper to do this?

 

[prt57]

Two things, I note you have changed the ABS pump but dd you do an ABS bleed using VCDS to operate the valves in the pump? This is how we solved the same problem on our Mk 4 Golf. Midland VW did this for us but any good VW specialist should be able to help.
With the bleed valve, have you tried totally removing it, putting your finger over the hole to retain the fluid then trying gentle foot pressure to eliminate any dirt in the valve causing a partial blockage? Don’t push too hard as you don’t want brake fluid squirting everywhere.

 

[NHN]

Using a gunson unit I would bleed all again, the caliper in question should bleed just the same as the others & as stated test the feed pipe for liquid movement when disconnected from the caliper.

 

[sdutton007]

Two things, I note you have changed the ABS pump but dd you do an ABS bleed using VCDS to operate the valves in the pump? This is how we solved the same problem on our Mk 4 Golf. Midland VW did this for us but any good VW specialist should be able to help.
With the bleed valve, have you tried totally removing it, putting your finger over the hole to retain the fluid then trying gentle foot pressure to eliminate any dirt in the valve causing a partial blockage? Don’t push too hard as you don’t want brake fluid squirting everywhere.

I've bled the ABS using VCDS several times.

I've just swapped the caliper for one from a working car and it still doesn't bleed using the 2-man method. I haven't had a chance to try the pressure bleeder yet but I expect the same result.

I removed the bleed valve and the fluid runs out as I'd expect. And I took the flexi hose off the caliper, it dripped fairly fast as I'd expect. So I really don't understand how the fluid is getting to the caliper but not through the bleed valve (without a pressure bleeder).

 

[prt57]

I am perplexed as well which is no help to you!
In theory, you push the brake with the nipple shut and it pushes out the piston onto the pad. Open the nipple and instead, fluid should come out the aperture and the piston should not move.
Have you had the car on an MoT brake test rig to see if there is a braking imbalance between the two front wheels? You would expect this to be occurring if fluid does not come out of the nipple when 2 man bleeding.
the calliper does not “know” the difference between you trying to bleed it compared to braking on the road.

 

[sdutton007]

I am perplexed as well which is no help to you!
In theory, you push the brake with the nipple shut and it pushes out the piston onto the pad. Open the nipple and instead, fluid should come out the aperture and the piston should not move.
Have you had the car on an MoT brake test rig to see if there is a braking imbalance between the two front wheels? You would expect this to be occurring if fluid does not come out of the nipple when 2 man bleeding.
the calliper does not “know” the difference between you trying to bleed it compared to braking on the road.

No, the car hasn't moved off my driveway for 19 months and would be dangerous to drive to an MOT. I could pay to get it transported but I don't think it would give me any answers.

Tomorrow, I'm going to take the caliper off and try to see if the piston moves when the pedal is pressed.

 

[prt57]

A good plan. Put it back on the calliper carrier without the pads in but put something between the disc and the piston so you can see it move but not too much that the piston pops out of the calliper.

 

[Harvey]

Have you replaced the flexi line to the caliper? It might have collapsed/deteriorated inside the flexi.
Remove caliper feed pipe from ABS & at the caliper & blow down/up the line to ensure there are no blockages.

 

[sdutton007]

Have you replaced the flexi line to the caliper? It might have collapsed/deteriorated inside the flexi.
Remove caliper feed pipe from ABS & at the caliper & blow down/up the line to ensure there are no blockages.

I haven't but it's worth a try. Will update once I've tried this thanks

 

[sdutton007]

I've had a thought - my understanding of the master brake cylinder is that it has 2 separate pistons inside - presumably one for the FL & RR brakes, one for the FR & RL brakes...

It's massively unlikely but if all 3 of my master brake cylinders had a faulty seal/piston, could this cause my problems?

TBH, I wish I could just rip out the entire badly-designed Audi brake system, replace it with Toyota stuff and just ignore all the ABS/ESP codes

 

[nluk100]

Did you check or replace the actual brake servo or just the brake cylinder as I watched a video the other day of an internal leak within the servo itself with no other obvious issues with the braking system?

I know this would usually affect your entire braking system, but you seem to have checked or replaced everything else.

The only other thing could be a kinked or crushed pipe from the abs pump to the wheel well that's having the issue?

 

[sdutton007]

Did you check or replace the actual brake servo or just the brake cylinder as I watched a video the other day of an internal leak within the servo itself with no other obvious issues with the braking system?

I know this would usually affect your entire braking system, but you seem to have checked or replaced everything else.

The only other thing could be a kinked or crushed pipe from the abs pump to the wheel well that's having the issue?

I replaced just the brake cylinder, not the servo.

I'm no expert in brake servos, but doesn't it just boost the brake pressure when the engine is on? With the engine off, it shouldn't have any effect, should it?

I would replace it anyway (since nothing else seems to fix the issue), but it seems to be a MASSIVE job involving dropping the engine a few inches

 

[nluk100]

I replaced just the brake cylinder, not the servo.

I'm no expert in brake servos, but doesn't it just boost the brake pressure when the engine is on? With the engine off, it shouldn't have any effect, should it?

I would replace it anyway (since nothing else seems to fix the issue), but it seems to be a MASSIVE job involving dropping the engine a few inches

I would check the pipe between the abs pump and the wheelwell first - maybe run a new temp line straight from the pump to the caliper to see if there is an obstruction.
With regards to the servo, there is a seal at the back of the master cylinder that can leak fluid into the servo regardless of if the engine is running or not - however, as I said this would normally affect all 4 corners.

 

[reverseproxy]

I replaced just the brake cylinder, not the servo.

I'm no expert in brake servos, but doesn't it just boost the brake pressure when the engine is on? With the engine off, it shouldn't have any effect, should it?

I would replace it anyway (since nothing else seems to fix the issue), but it seems to be a MASSIVE job involving dropping the engine a few inches

It's a force multiplier iirc although it's a bit strange how only 1 side affected as if you still have air or blockage in it. I highly doubt 3 master cylinders have gone bad. I can't recall which way around it is, but normally pedal should go hard pumping it with engine off (I believe it sinks on Diesels due to design), but should go hard with engine running on both vehicles. Possibly something to do with hydraulic pressure?

I'd be tempted to bleed the entire system with VCDS with the ABS pump method, doing it in the correct order (RR, LR, FR, FL), that should get rid of any air in the system..

EDIT - Might want to try nluk method first, run a new line to the caliper from the pump, quickest way to rule out a bad line.

Have you looked into the pressure sensor readings on the pump with VCDS too?