Rear brake calipers wont bleed

I have a 1J Mk4 GTI 1.8T AWP with ASR (no ESP) with a lot of odd brake problems that i'm very deep into with time money and effort.Most of them i've "solved." (I think. I haven't gotten to road test the car much) There is one I have yet to figure out:

Symptoms: When I open the rear brake caliper bleed screws and pump the brakes, no fluid comes out and the pedal feels hard. This has changed over time from not being able to bleed one rear caliper to now being unable to bleed both rear calipers.

Solutions: New soft lines for all four corners. New hard lines going from the ABS unit to the rear soft lines. New bleeder screws. Took rear calipers off and tested flow from line entry to bleed screw with compressed air (caliper flowing correctly). Cycled ABS pump with VCDS block 03->output tests. Went through procedural steps. Took ABS unit off and blew air through ports, everything flowed symetrically and without issue. Purchase low mileage ABS unit and install both the module and fluid block. Cycle ABS with VCDS block 03->output tests again. Unplug battery and ABS unit plug (Resets ABS?) Remove lines going from master cylinder to ABS fluid block, blow through with compressed air (lines flowing correctly). Reinstall lines, pump brake pedal to test master cylinder flow (fluid comes out in bursts from both lines, seemingly sucking air in when pedal reaches the bottom of its stroke).

Recently replaced parts: New calipers, all four corners. New brake pedal switch. front right new ABS ring. New rear hubs/ABS rings. New ABS sensors all 4 corners.

I'm unsure of what proper master cylinder fluid flow looks like and what improper master cylinder fluid flow looks like. I'm unsure of how to test a master cylinder for correct operation. A few friends have advised replacement of the master cylinder and doing a bench bleed of the new unit. This is the last option and if this does not solve the issue, I'm unsure of what to do next. AS of now, I can only bleed the front calipers successfully.

Any alternative solution ideas or further understanding of how the ABS/brake system operates either electrically or hydro-mechanically would be greatly appreciated.

Any vacuum pump or similar...

Tried that. Applied 25inHg vacuum, opened caliper bleed screw. No fluid flowed into the canister. Left vacuum on for 5 minutes, repumping to maintain vacuum.

If the line is, for example, the right rear end open, the oil in the tank has, it must come even in free-fall. If this is not the case, there is still an obstacle.

My first assumption would be the main cylinder.

This procedure details how to bleed the ABS pump when replacing brake fluid. This procedure is NOT necessary unless your ABS pump has gone completely dry. For normal brake work including fluid replacement, hose replacement, pad replacement, etc. this procedure is NOT necessary. Also, this procedure does not take the place of good ol' fashioned brake bleeding with one person operating the pedal while the other person opens the bleeder valves one by one and collects the discharged fluid.

There are certain test conditions that must be met before doing this procedure:

Vehicle must on a lift or safe jack stands with no wheels touching the ground

Ignition should be in the "On" position

Procedure for Bleeding the ABS Brake Pump on VW Golf, GTI, Jetta:

[Select]
[03 - ABS Brakes]
[Basic Settings - 04]
Group 001
[Go!]

This will trigger the ABS pump to turn on for about 10 seconds. You should hear the fairly loud pump clicking during this time.

Click the [Done, Go Back] button and you're all set.

Click to expand...

ok thanks. I'll try that different pathway in VCDS for running the ABS pump after i replace the master cylinder. I have a new one arriving today.

I’ve unplugged and checked all the hardlines for obstructions. There are none and flow air freely through them while disconnected to the ABS unit. I replaced the master cylinder after bench bleeding. I am out of options. The rear calipers still will not bleed.

I ran
[Select]
[03 - ABS Brakes]
[Basic Settings - 04]
Group 001
[Go!]

And this got the pedal to not feel firm anymore when trying to bleed the rear brake calipers by foot which I am very confused at as the prompts only asked to crack the FR-FL bleeders.....
Fluid and air still doesn't seem to come out of the line. I'll try bench bleeding the master cylinder again. I'm going to buy a pressure bleeder as per the Bentley and try that.

aside from:
[Select]
[03 - ABS Brakes]
[Basic Settings - 04]
Group 001
[Go!]

-and-

[Select]
[03 - ABS Brakes]
[Output Tests - 03]
[Go!]

Are there any other selections in the block that can be used to assist with this issue? Success is still vauge.

Does oil flow from main cylinder to ABS unit?

Gravity bleed zone by zone & until obstruction located by isolation and not with air or vacuum!

Does oil flow from main cylinder to ABS unit?

Click to expand...

Yes. But this was checked with lines disconnected from the ABS unit. Not sure how to check for this with lines connected.

Gravity bleed zone by zone & until obstruction located by isolation and not with air or vacuum!

Click to expand...

I did this for 20 minutes, no fluid or air will flow from any part of the brake system past the ABS unit. I gravity bleed every part of the brake system, removing each component from the back of the car to the front until everything was disconnected except the ABS.

this concludes there is an issue with BOTH of the ABS units i have on hand in very similar manners. Almost identical symptoms. One ABS unit cannot bleed the rear right corner (and all fluid flowing components up to said caliper), the other ABS unit cannot bleed the rear right corner or the rear left corner (and all fluid flowing components up to said calipers). Contacting multiple companies claiming to rebuild ABS units about this, they say that it is incredibly rare for two ABS units to fail simultaneously in almost identical ways.

I attempted to disassemble and clean the ABS unit valves and only managed to destroy them trying to remove them. I also bench performed some pneumatic airflow tests on my old ABS unit. I blew compressed air through the ports that flow in from the master cylinder. Air comes out of the front right, front rear, rear right... but not rear left on my original (250,000 miles ABS unit). The other ABS unit has not yet been bench tested as it is currently on the car.

I've studied the [03 - ABS Brakes] section and performed all active prompts within the section. One of the stranger things in [Select]
[03 - ABS Brakes]
[Basic Settings - 04]
Group 001
[Go!]
Is that the prompts seem to just loop over and over. Press and hold pedal, wait 10 sec..., open FR+FL bleed screws, wait 10 sec..., Push pedal 10 times bleed screws close......sometimes in different orders, over and over. Is this supposed to prompt something else? It seems broken.

I think I'm at the point where I need to take a chance on either another used ABS unit, A re-manufactured one, possibly or a brand new one which is ridiculously expensive and worth more than the entire car.
Any alternative ideas would be appreciated.

I had a MK5 ABS unit that wouldn't bleed or flow fluid to the right rear. I believe one of the normally-open valves to that wheel jammed in the shut position.

Mine failed after a "spirited" take-off, around a right hander, which activated the stability and traction - I think it jammed while performing it's duties, and never unjammed - the valve that closes flow to that wheel jammed in the shut position. I never had a code, but the car would pull under brakes (the extra fluid from the split cirrcuit going to the diagonally opposite wheel)

I changed over the hydraulic block to a spare one, that I had kept immaculately clean (actually the original off my car - I hadn't realised I could keep the block, when I changed my ABS to the 4WD version). That solved my issues.

I suspect you have a similar problem.

People don't clean around there ABS connections properly, and then don't seal the holes with clean bungs - a TINY bit of grit can cause mayhem.

You could running the abs output test (bleed procedure)? At least the valves will be actuated then.

I did that 3 times today. And the basic settings procedure 3 times. I finally maxed out basic settings prompte. It ended with “partial venting finished.” Never once prompting to open the rear bleeders. Has anyone ever read the prompts for a mk4 abs/edl/asr basic settings bleed test? I’d like to know exactly what other people are seeing.

Also, others suggested that i take the car out and hammer the living crap out of the abs system. I did that until i completely lost all brake function, limped home with the e-brake and rebled. Luckily i was only a block from home. Not sure if i overheated the pads or what. Bled all four corners. The rears seemed to bleed with a bit more effort than they should. I managed to squeeze some fluid out of them. Theres still air in there. Then i ran out of my 4th can of fluid (yes ive been recycling the fluid that comes out, but not all of it can be reused (me letting some sit out overnight after working 10 hrs then coming home to this mess, i forget to pour fluid back in the can sometimes)).

I’ll try the same procedure again tomorrow after bleeding again. Maybe i wont lose all brake function like today. Not sure what happened. Came back, inspected for leaks, not even a residue anywhere. Fluid level was where it was when i left. Perhaps some air got purged from the ABS or master cylinder. This method is extremely dangerous but it seems like its getting some results.

Old school way (with person in car and pumping) to bleed is:

1. Rear right end (longest way from master cil.)

2. Front left

3. Rear left

4. Front right

This method work even with gravity. In special case, with broken bleed screw, piston move max forward, close to the edge, fill inside with fluid and with tool move piston full backward. Fluid with air must purge through reservoir.

But if we haven't fluid flow to rear usually tell us about some problem. :/

I solved the issue by replacing the 3rd bad ABS unit with another one from the junkyard being careful to snip the hoses instead of undoing them at the yard, installing the unit and NOT doing anything through VCDS. I got lucky. I still have no clue as to why all of the ABS modules i installed prior were faulty.

If i have time, i will hydraulically seperate the solenoids and their internal parts and inspect them for corrosion.

Brake fluid exposed to air deteriorates rapidly. If someone takes a caliper or a master cylinder and leaves everything hanging, the pump will be dead in weeks/months.

MasterTommy said:

Brake fluid exposed to air deteriorates rapidly. If someone takes a caliper or a master cylinder and leaves everything hanging, the pump will be dead in weeks/months.

Click to expand...

Indeed, and that is why brake components (those that hold fluid come sealed with SELL BY dates.