Brakes Bleeding Problem: pedal is still soft

[alexperkins]

Very strange. I wonder if your brake servo has issues

Is the pedal rock hard when the engine is off

Another thing to consider is upgrading the master cylinder as it might not be big enough for the pistons on those calipers. What are those brakes off? I assume R32/TT etc

[Jay-Jay]

I tested the brake-servo and its hose connected immediately under the throttle body: everything is ok.

When engine is off, if I press the brake pedal 3/4 times, it gets hard as a rock. Then, if I keep pressing on the pedal, it stays there even after 20/25 min. (yes, I've stayed there pressing the pedal for that long time to be sure my master cylinder is in good condition).

I also thought about the master cylinder being undersized. But then I discovered that piston size of my calipers is the same of my old original calipers (54mm).
Plus to this, the calipers I have installed are specific for Polo 6C GTI: they are the same ATE calipers installed on all Polo 6R/6C with 288/310mm front​ discs (GTI and R WRC included).
According to ETKA, the master cylinder's size (22,2mm) is the same for every Polo untill MY2011 (Bosch 8.2i). Starting from MY2012 (Bosch 9.0i) Polos with 310mm front discs (only GTI and R WRC) have a little bit bigger master cylinder (23mm), but those with 288mm front discs (same calipers as mines) still have my same master cylinder.

So, in the end my conclusion is that the master cylinder I have is enough for my calipers.

I have tried the brakes on the 6R GTI MY2011 of a friend with 310mm front upgraded discs and my same calipers: its brake pedal is extremely stiff.

[iichel]

I think the Polo GTI 310mm/WRC do have a larger master cylinder... but you'll have to look that up in ETKA. But I have no issues with the 'small' master cyclinder.

[Jay-Jay]

Yes, as I have just written, starting from MY2012 only GTI and R WRC have a little bit bigger master cylinder (23mm).

But the 22,2mm version (mine) is still used with my same calipers for those Polo with 288mm. So, it's not a problem of master cylinder's size.

[alexperkins]

Wonder if there’s a fault in your ABS unit then considering everything else

[Jay-Jay]

That's another thing I thougt about.
With VCDS I tested the ABS unit with the "Output Test" function and everything seems fine.

Last year, when I had the suspect to have a faulty ABS unit, I ordered online two used ABS unit in working condition. I've never installed them, since I am afraid of disconnecting the hydraulic unit from the brake system hoses and install a dry unit, then start again with the bleeding process, with the risk of making my problem even worse. [emoji18]

I guess that, if ABS, ESP, ASR, XDS, steering angle sensor, lateral and longitudinal acceleration sensor, brake pressure sensor, Hill Hold Control and TMPS work perfectly fine, my ABS unit (the heart of all these functions) should be ok too.

Moving by exclusion of possible causes, I'm afraid that there's only one subject that remains to be deepen: the ABS unit bleeding process!

Between all the things I tried, it is the only one that made some difference in the brake pedal feel, but I suspect I didn't correctly the procedure.

Is there someone who did it and can share his/her experience?

I asked in a VW workshop for an ABS Bleed, but I received the answer that their mechanics are allowed to do this procedure only when the ABS Unit has been replaced or accidentally got dry. So they refused to do it on my car and told me to repeat the normal bleeding procedure untill I get the brake pedal feel I want.

[iichel]

tell them you replaced it, lol

[Jay-Jay]

I already did it! They told me: "we can't guarantee the result of the procedure, because it's not a replacement did by us with an original new ABS Unit, but by you with an ABS Unit from unknown origins. Who knows if that ABS Unit is defective?".
They also complained about me having modified the front brakes, suspecting they could be defective too.

In the end, I understood that they are available to do the ABS Unit Bleeding procedure only if I order the part from them and let them install it on my car (in less words, they want to earn money).

But I'm not stupid and I already abandoned the idea of trusting them.

In the meanwhile I have a good update.
Yesterday I tried another bleeding procedure:
- I connected the pressure bleeder;
- with VCDS I did few cycles of ABS Unit activation;
- then I opened the bleed nipple of calipers in the ElsaWin order.

Very little air bubbles came out of only the rear calipers, especially the rear right one (which is the furthest from the ABS unit).
I repeated this entire procedure twice. The second time two little bubbles came out of the front right caliper too.

Now the the brake pedal's travel has become very short! I would say It's the half of what it was before! And most importantly, the pedal is not soft like before! The more I press it down, the more it gets stiff!

Yesterday night I drove some km to try this new pedal and adapt to its new feeling.
It rained a lot, so I wasn't able to test correctly the ABS, but I had the impression it is now even better than the original brake pedal I had before all the modofications I did to the brakes!
It seems like it is harder to press and with a shorter travel: plus to this, it's a lot more reactive (I guess, thanks to HEL Performance hoses).
During my drive I noticed that the pedal feel was always constant: it remained always the same during all the trip.

My guess is that, thanks to the constant pressure applied to the brake circuit by the pressure bleeder, while I made the little ABS valves open and close repeatedly (activating the ABS Unit with VCDS), I succesfully removed air bubbles trapped into the ABS Unit. I don't know if I should repeat again this process, but now I am really satisfied with the pedal feel.

As soon as I can, I'll compare my brake pedal with the one of my Polo's friends.

I'll keep you updated!

[Polonut]

I cannot see any mention of rear brakes. Are they Disk or shoes?

If they are shoes.... have you played around shortening the handbrake cables?
You may think why is this guy asking this question.... there is a reason based upon a full automotive apprenticeship and some 25 years in the workshop.

Also, when you fitted a new master cylinder, did you check the required push rod clearance (this used to be adjustable years ago - but on a modern car this may be fixed)

[Jay-Jay]

My Polo 6R has rear discs brakes (232mm with Lucas calipers).
From what I saw with my eyes, the push rod clearance is not adjustable on the Polo 6R.

After few days of driving I can confirm the brake pedal feel is now almost perfect: very reactive and not soft as before and finally with a short travel.
Maybe when I'll replace front brake pads, I'll bleed once more time the front calipers with the piston completely compressed.


There's a thing I noticed.
When Hill Hold Control activates, the brake pedal becomes very hard. But if I press it down, I can press it a little bit (let's say, about 1cm of travel), then it becomes really impossible to press it more. Is it normal? I remember that when I had original front brakes, the brake pedal was immediately impossible to press while HHC was active.
Could it mean there's still a bit of air in the ABS Unit?