New fluid into brake lines and ABS module

[workingonit]

My vehicle has a softer pedal than I would like. I will try to replace all the brake fluid to see if this gets rid of the soft pedal. I have no idea of the age of the brake fluid so a service is mandatory anyway.

I am not replacing any components, so no mass air is being introduced into the system.

This little job has prompted me to activate and code the Nanocom after sitting in its box in the cupboard for nearly 2 years. Found the 'Power Bleed' and 'Modular bleed' options.

Read the web for the procedures. Some conflicts in procedures. In simple terms it seems I should do the following three steps.

Step 1.
I should first do an old fashion manual bleed of the four brake lines by pumping the pedal to move fresh fluid in and move old fluid and any air out. This seems to be the only thing some people need to do to get a hard brake again and is their only introduction of new fluid to the system.

This step in itself does not seem to move old fluid and any air out of the the ABS module.

Step 2.
With all caliper bleed nipples closed, foot on the pedal, activate the ABS 'modular bleed'. This seems to 'circulate' the fluid in the ABS module and at the very least move air out of the ABS module into the 4 brake lines.

Instructions seems to implied that 'circulation' moves new fluid into the ABS module, but I do not understand how. This is my area of greatest confusion.

How does the 'modular bleed' option force the old fluid out of the ABS module and into the brake lines when the brake lines are already full of new fluid and the bleed nipples are closed? In some threads others have asked should the bleed nipples actually be open in this procedure - invariably the answer is no.

How can I be sure I'm getting new fluid into the module and not just floating air bubbles out while retaining the old fluid?

Is this a bit like changing automatic transmission fluid - you can never get all the fluid out of the torque converter so a fluid change is never 100%?

Step 3.
Clear all four brake lines again, as in step one, but using the 'power bleed' instead of pumping the pedal.

Is a power bleed simply a substitute for the old manual pedal press method? The instructions imply some difference.

[ozscott]

From memory the power bleed is the substitute for a second person doing the brake pedal work. Allows for single person bleeding. Cheers

[Pedro_The_Swift]

The "power bleed" just activates the ABS,, it works fine,
but you'll never get a D1 brick-like brake pedal,,
D2's stop fine, just have a longer pedal travel than most...

gravity also works.. a bit slower, just keep an eye on the res.....

[workingonit]

The way I read it. We commonly think we need to expel air by expelling brake fluid, and this method is used in step 1 and 3. Air can be removed from closed systems by circulating the fluid as in step 2 for the ABS module, floating the bubbles to a trap, no fluid is expelled (although they might be exchanged) - the trap in this case are the 4 brake lines - some systems like engine cooling circuits might get the bubbles to float back to the reservoir.

Everything I have read indicates step 1 should be done pumping the pedal while opening/closing the bleed valves until there is new fluid coming through and no more air bubbles. You can also use powered tools that push brake fluid through the brake lines while again opening/closing the bleed valves. It is always specified that the ignition be off so that the ABS system is not active, therefore no use of the 'power bleed', and also no activation of ABS valves. You can understand why people see the power bleed being automation of the manual system requiring one person instead of two, but none of the 'official' literature recommends power bleed in step 1. If for example I have to bleed my D1 brakes by myself I use a stick between the seat and brake pedal while ducking around to open/close the bleed nipple - a lot of in and out of the vehicle but it works.

Lovemyrangie gives some useful insights into step 2. D2 ABS Modulator power bleed (https://www.aulro.com/afvb/discovery-2-a/131263-d2-abs-modulator-power-bleed-2.html). Pushing and holding down the brake pedal in step 2 opens the reservoir fully to the ABS unit as it circulates fluid. It implies to me that this process migrates air bubbles to a point where they float into the 4 brake lines, without actually pushing new fluid into those 4 lines (remembering that the 4 lines are already full of new fluid and the nipples are closed) - that the old fluid in the ABS modulator is mixed with new from the open reservoir as the circulation step occurs. It's one thing to get air out of the ABS modulator, and another to get sufficient fresh fluid into the modulator to meet technical requirements, reminding me of changing auto trans fluid where it is only about a 70% change. As one writer said if you change your fluid regularly you should be OK. Again, I can see why some people open bleed valves when performing the modulator bleed - but 'official' instructions always require the bleed nipples be closed in this step - and Lovemyrangie seems to be saying surge is reduced by having nipples closed in step 2.

In step 3 you do not use the brake pedal at all, you do not pump fluid with it, nor use it to open the reservoir fully to the ABS unit. My assumption is that sufficient fluid can be pulled from the reservoir by the power feed pump, sufficient to fill the 4 brake lines, one at a time, opening and closing the bleed nipples. Apart from expelling the air floated to the 4 brake lines in step 2 I'm guessing this also expels some of the fluid that is now a mix of old and new as a result of step 2. I'm uncertain as to whether this step improves on the exchange of new for old in the ABS block itself.

[jwb]

I found I got a great result and a really firm pedal when I replaced the (probably original) brake hoses with stainless braided type, replaced the two flexible hoses to the rear lines, new fluid and thorough bleeding. Did it all at once and very happy with the result.