Brakes Bleeding Problem: pedal is still soft

[Jay-Jay]

Hi everybody,
2 years ago I installed bigger front brakes (from 256mm to 310mm). The brakes I installed were new: not only calipers, but also rotors and pads.
After installing the new calipers, I bled the entire brake circuit.
After that I immediately noticed that the brake pedal was too soft and with a longer travel.

During the last 2 years I tried many things to fix this problem, without any success.
I lost the count of how many times I bled the brakes...

This is a short list of what I did:
- replaced the brake hoses with stainless steel ones (HEL Performance);
- replaced brake fluid with DOT 4 LV (ATE SL6);
- bled the ABS unit with VCDS (but I'm not sure if I did it right )
- replaced the master cylinder with a new one;
- replaced front calipers again with new ones.

I bled the brakes with the normal procedure (one person press the pedal and another opens the caliper's nipple) and also using a pressure bleeder.

According to ElsaWin, the pressure bleeder should be pressurized at almost 2bar (I never exceeded 1,9bar) and the correct bleeding order is:
- rear left
- front left
- front right
- rear right

The only benefit I obtained after all these tries is a very reactive pedal, but the long travel and the softness is still here.
This softness is not always the same, but changes while driving. So it seems like there's still some air trapped somwhere inside the brake circuit.

With VCDS I noticed that if I don't press the brake pedal, the brake pressure fluctuates between 0,00bar and 0,33bar: it seems like there's some air that's trying to expand, applying a very little pressure on the brakes, without touching the pedal.
I checked the same thing on the Polo of many of my friends who never unplugged calipers and the brake pressure is -1,33bar (gasoline engines) and 0,00bar (diesel engines).
Considering that diesel engines have a dedicated vacuum pump for the servo-assistance, I guess I have to take into consideration only the value of gasoline engines: -1,33bar.

It's clear that something is wrong on my car, but at this point I don't know what else I could try.
Maybe I should try again VCDS ABS Bleeding procedure, but I cannot understand how it has to be done correctly.

Any suggestion is really apreciated. I'm really desparate.

[iichel]

Hey Jay Jay, what kind of method do you use? Pressure bleeding and pumping the pedal, I read.

I did the same upgrade, and I'm pretty happy. 310mm front 256mm rear and SL6 fluid.

First I used a vacuum pump to suck out the fluid, but now I bought a Gunson Eezibleed and I'm a lot more satisfied with it.
It's not very expensive either: https://www.amazon.co.uk/Gunson-G4062F- ... B001KTFIWA

It is very important to make sure the nipple is closed properly and is not sucking in air. I fcuk'd that up a few times, and had a spongy pedal.

I agree a soft pedal is very annoying.

[Jay-Jay]

Hi iichel,
yes, I tried pumping pedal and pressure bleeding procedures and also both together.

The pressure bleeder I have is very similar to yours. Mine is the Sealey VS0205, which gets the pressure from a tyre. I made a little modification to it, so now it can reach 1,8/1,9bar without problems. It is very usefull: I succesfully bled my clutch thanks to it.

Do you know what's the correct way to bleed the ABS Unit with VCDS?

[iichel]

I'm sorry but I don't know, the normal bleeding was sufficient. Is there maybe a label in the basic settings? Unfortunately ELSA only mentions the VAS tester, nothing about VCDS (as to be expected)

[Jay-Jay]

I tried this: ABS Brakes --> Basic Setting --> Group 001 --> Go!
Following the on-screen instructions doesn't help me so much.

The procedure is divided in 2 phases:
- 1st phase: front calipers (ABS pump runs for a total of 22 times)
- 2nd phase: rear calipers (ABS pump runs for a total of 14 times)

During both phases VCDS says to press the pedal 10 times and it wants me to open the nipple of both calipers of the same axle at the same time.

I instead opened the nipple of only one caliper per phase.
I started from front left caliper, then restarted the procedure and moved to the front right caliper.
Then I moved to the 2nd phase and started from the rear left caliper and then moved to the rear right caliper.

In the 1st phase, I noticed brake fluid gets out of front calipers only during the last 8 ABS pump activations. During the first 14 activations, the ABS makes a louder noise, the brake pedal moves up and down, but no brake fluid gets out of the nipples. It seems like the ABS pump creates sort of a vacuum inside the circuit.

At first the brake fluid that came out of the front calipers was full of very little air bubbles. After 2/3 cycles, brake fluid started to flow out without these little bubbles.


I was pretty sure to have found the cause of my problem, but the brake pedal is still soft.

[iichel]

i'm probably asking the obvious, I know you're good at this, but you are sure that the new material is OK and mounted correctly? No leaks, the calipers are good and not leaking?
did you code your ABS unit correctly for the new brake sizes?

[Jay-Jay]

Yes, everything is installed correctly and no leaking.
The brake fluid stays almost always at the same level (it gets lower only because of pads and discs consumption).

The coding of my ABS Unit is specific for the 288mm front disks, because Bosch 8.2i doesn't support the coding for 310mm disks.
But I don't think it could be the cause of my problem, because many of my friends with a 6R GTI MY2011 upgraded from 288mm to 310mm without changing the ABS coding (they couldn't anyway) and don't have problems.

The strange thing is this:
there's always some air bubble that suddenly gets out of a caliper, even if I bled the brake system recently.

I really can't understand where this air comes from: if it's sucked in by a nipple which doesn't seal perfectly (but I should notice a leak, no?) or if it's produced by humidity condensated and boiled (which is strange, because the brake fluid is new and shouldn't have already absorbed this much humidity).

This situation is making me mad and crazy.

[iichel]

I agree it's strange, but I don't know how to solve it... i'm very sorry

[Jay-Jay]

I thank you anyway, iichel!

If someone else have some other suggestion, is really appreciated.

[alexperkins]

Have you bled the clutch?

[RUM4MO]

I only know that some callipers, when they are bolted onto the car do naturally leave an air pocket void at their highest point, the DIYer way round that is to hold the calliper so that the bleed valve is at the top when bleeding each calliper, obviously placing something in the calliper fist to keep the pistons fully retracted. Other than that only the obvious point which is far too late for you, ie never ever drain the system, or let the system run dry!

[Nogster]

Hi there it might seem like a daft thing to say but make sure the calipers are on the right side and the bleed nipples are at the top,I’ve know it where someone has fitted the caliper on the wrong side and the nipple was at the bottom,This then Trapps the air at the top of caliper and you can’t get it out.

[Jay-Jay]

Have you bled the clutch?

Yes, I did it.

I only know that some callipers, when they are bolted onto the car do naturally leave an air pocket void at their highest point, the DIYer way round that is to hold the calliper so that the bleed valve is at the top when bleeding each calliper, obviously placing something in the calliper fist to keep the pistons fully retracted. Other than that only the obvious point which is far too late for you, ie never ever drain the system, or let the system run dry!

I've never drained the system or run it dry!
I already bled all the calipers with the nipple at the highest point and the piston fully retracted, to be sure no air was trapped inside their cylinder. Nothing has changed.
I'm convinced some air is trapped inside the ABS Unit. I don't know how the hell it got there, but I need to understand how to expell it.

Hi there it might seem like a daft thing to say but make sure the calipers are on the right side and the bleed nipples are at the top,I’ve know it where someone has fitted the caliper on the wrong side and the nipple was at the bottom,This then Trapps the air at the top of caliper and you can’t get it out.

I installed my calipers correctly with the nipple at the top. Otherwise I wouldn't have been able to connect the brake hoses.

I want to thank you all for your help.
I really hope we'll find the solution!

[alexperkins]

Did you prime the calipers with brake fluid before you fitted them to the car?

[Jay-Jay]

Yes, I did!