Thread: Two Brake callipers not bleeding the entire brake pedal stroke - faulty ABS units?

I suspect that either this new master cylinder is faulty and fluid is escaping past the internal seals when vacuum boost assisted, or perhaps the ABS unit has now also got issues.

If the travel of the pedal is longer than it was in previous cases then IMO only the master cylinder can be the problem or there is air left in the system somewhere...i can't see any logic in how the modulator can cause that without any leak. I've seen the same bleeding procedure you quoted in a Wabco document and IMO it's not the best way, the sequence described in RAVE has more logic for me. A good move would be to find a place to activate the HDC and let it work a bit then see if something changed or not.

Originally Posted by twr7cx

I find this strange as the issue is diagonally across the vehicle effecting one front and one rear caliper. The master cylinder looks to have the two front callipers and two rear paired up for the combined outlets, after this the ABS module looks to have four individual outlets for each caliper. Could the issue therefore be with the ABS module and some sort of internal blockage?

How did you assess that the issue was occuring diagonally? From what I understand the two chambers in the cylinder activate the front and back separately and maybe one kicks off stronger before the second kicks in (not sure since its been to long since I work on mine, hope someone can confirm). It's seems unlikely that the piping from cylinder to ABS (or out) could be crossed. This diagonal issued that you note doesn't make sense? Would be interested on how you determined this?

(Just for info. When bleeding a cylinder the pedal travel goes way deeper than normal. With an old cylinder there can be rough rust past the normal travel and you can destroy the seals during bleed. Which may have happed when you lost the fluid.)

Originally Posted by sierrafery

If the travel of the pedal is longer than it was in previous cases then IMO only the master cylinder can be the problem

This is my belief. I just want to make sure before trying to do an international warranty claim...

Originally Posted by sierrafery

I've seen the same bleeding procedure you quoted in a Wabco document and IMO it's not the best way, the sequence described in RAVE has more logic for me.

I'll look up the Rave sequence now. I'm not familiar with it.

Brake system bleeding

$%70.25.02


Bleeding of the brake system can be carried out using the procedures given on TestBook, or by following the manual procedure given below.

WARNING: If any components upstream of brake modulator, including the modulator itself are replaced, the brake system must be bled using the procedure on TestBook/T4, to ensure that all air is expelled from the new component(s).

WARNING: Do not allow brake fluid to come into contact with eyes or skin.


Bleed


NOTE: This procedure covers bleeding the complete system, but where only the primary or secondary circuit have been disturbed in isolation, it should only be necessary to bleed that circuit. Partial bleeding of the hydraulic system is only permissible if a brake pipe or hose has been disconnected with only minor loss of fluid.

1. Do not allow fluid level in master cylinder to fall below 'MIN' mark during bleeding.

2. Do not fill reservoir above 'MAX' level.

3. Raise front and rear of vehicle.

WARNING: Do not work on or under a vehicle supported only by a jack. Always support the vehicle on safety stands.

4. Check all pipe and hose connections are tight and there are no signs of leakage.

5. Top-up fluid level in brake reservoir to 'MAX' mark.

WARNING: Do not allow dirt or foreign liquids to enter the reservoir. Use only new DOT 4 brake fluid from airtight containers. Do not mix brands of brake fluid as they may not be compatible.

6. Attach bleed tube to the bleed screw on front brake caliper on the passenger side, submerge free end in a clear container containing brake fluid.

7. Apply pressure to brake pedal several times, then apply steady pressure.

8. Loosen bleed screw to release brake fluid and air. Allow pedal to return unassisted.

9. Depress brake pedal steadily through its full stroke and allow to return unassisted. Repeat procedure until a flow of clean air-free fluid is purged into container then, whilst holding pedal at end of downward stroke, tighten brake caliper bleed screw to 10 Nm (7 lbf.ft).

CAUTION: Ensure the fluid in the reservoir is maintained between the minimum and maximum levels throughout the bleed procedure using new brake fluid.

10. Top-up brake fluid level to 'MAX' mark.

Bleed sequence RHD
1 Passenger front
2 Driver front
3 Passenger rear
4 Driver rear

11. Working in the sequence illustrated, repeat steps 5 to 9 on remaining calipers.

WARNING: Braking efficiency may be seriously impaired if the incorrect bleed sequence is used.

12. Apply brakes and check for leakage.

13. Remove stand(s) and lower vehicle.

14. Road test vehicle. Check brake pedal for short firm travel when brakes are applied.

How do you find this process better? It just states "system must be bled using the procedure on TestBook/T4" without providing any specifics on the process?

Originally Posted by sierrafery

A good move would be to find a place to activate the HDC and let it work a bit then see if something changed or not.

I have plenty of private land to do that on, but my concern is that it's a 8km drive on public roads from the workshop to access it. While the brakes do stop the vehicle they just don't fill be with confidence about driving on public roads. I have activated the HDC for approximately 500m on a steep hill right outside the workshop but it had no change.

Originally Posted by Mort2

How did you assess that the issue was occuring diagonally?

Did you read any of the posts on the first page? I worked out that the issue was diagonally as I went around bleeding each caliper. The Driver Front and Passenger Rear (which are diagonal from each other) were not pushing out much fluid at all and only on the initial first bit of pedal travel. The other two which are diagonal to each other, functioned properly.

Originally Posted by Mort2

From what I understand the two chambers in the cylinder activate the front and back separately

Again, did you read any of the posts on the first page? This was my belief too, as this is how other systems I've worked with function. But as others pointed out in the earlier posts in this thread, it's not how the D2 system is. They're diagonal. Someone even put up a picture mapping out how it's all plumbed to give you a visual idea.

Originally Posted by Mort2

(Just for info. When bleeding a cylinder the pedal travel goes way deeper than normal. With an old cylinder there can be rough rust past the normal travel and you can destroy the seals during bleed. Which may have happed when you lost the fluid.)

I doubt it in this case. I originally bled it because there was an issue - so the issue existed before the bleed, hence why I don't think the bleed caused it.
After fitting the new master cylinder I bled it, it shouldn't have a normal travel at that point and be rough afterwards - if it is then the product is faulty.

Hi twr7cx,

When I had a 2001 TD5 D2 I did a brake bleed as the pedal had a longer travel than I thought it ought to have. Had trouble getting a firm short travel pedal til I used the Nanocom to do a modulator bleed (think thats what its called).
It pulsed the ABS circuit as commanded when I bled each line and I got some tiny air bubbles out.
I had a good and firm pedal after and the D2 stopped much better.
So, do you have access to a Nanocom or similar to bleed the ABS ?
Might be the missing link as you have a new master cylinder fitted.

Regards
GIL

Originally Posted by GIL

So, do you have access to a Nanocom or similar to bleed the ABS ?

I sure do. As per Post #17 I've run the Modulator Bleed and Power Bleed Functions of the Nanocom.

As far as I was aware though, both the Modulator Bleed and Power Bleed Functions are done with the bleed nipple closed off. The Modulator Bleed is done with your foot applying maximum pressure to the brake pedal while the Power Bleed is done 5 seconds afterwards with the brake pedal released. Is this incorrect?

EDIT:

According to the Black Box Solutions Nanocom Wabco SLABS ECU Guide < wabco-slabs http://www.nanocom-diagnostics.com/d...ad/wabco-slabs >:

WABCO SLABS - Diagnostic Capabilities (UTILITY)
Choice of functions that can be performed
ABS BLEEDING

• Power bleed: This function opens all of the ABS valves and powers the pump causing fluid to circulate around the system, thus removing any trapped air bubbles.
• Modulator bleed: This function opens the abs modulator valve and powers the pump causing fluid to circulate around the modulator system, thus removing any trapped air bubbles.

Doesn't provide specific guidance on the brake calliper bleed nipples, but for both does indicate that it's about causing the fluid to circulate around removing trapped air bubbles from inside the ABS unit which makes me believe the bleed nipple does not need to be open for these processes.
Some discussion about it in Disco 2 - Brake Bleeding With Nanocom Evo | LandyZone - Land Rover Forum . Initially it's suggested it's for bleeding through the nipples but later discussion in the thread overwrites that.

Hi twr7cx,

As I recall I had trouble getting the air out.
I do know I tried with the bleed nipples both open and closed, and also with a foot on the brake pedal while using the nanocom.
I don't remember which was the magic combination.
I DO recall how good the brakes were after the air was purged though.

Sorry the memory is not what it used to be.
GIL

[/FONT][/QUOTE]

I doubt it in this case. I originally bled it because there was an issue - so the issue existed before the bleed, hence why I don't think the bleed caused it.
After fitting the new master cylinder I bled it, it shouldn't have a normal travel at that point and be rough afterwards - if it is then the product is faulty.[/QUOTE]

Hi twr7cx,
Apologies, whilst I did read through I was having a particularly bad day and nothing was going in.....I shouldn't have engaged and usually check my facts before I do.

With the issues of rust I had as information only, I thought that if you had noticed a change in depth of pedal with the original cylinder that it might have poor seals. And subsequently the new cylinder needed to be tested as you proposed.

Regards, Mort2

Originally Posted by twr7cx

Initially it's suggested it's for bleeding through the nipples but later discussion in the thread overwrites that.

IMO you certainly need to open the nipples cos the valves which are opened during the power bleed are directing the flow toward the calipers when the pump is activated so the air has to get out somewhere