GOLFMK8
GOLFMK7
GOLFMK6
GOLFMKV

Inconsistent and spongy brake pedel feel.

woodardhsd

Ready to race!
Location
NC
Car(s)
2020 GTI S DSG
Wow.

Whats the condition of the bleed screws? Are they potentially crusty with old fluid and leaking some air back in when you close them? You do have at least an 8" hose hanging off the door bleed screw when bleeding?
Bleed screws look just fine. There's no real corrosion or rust on any part of the brakes or suspension. It's a 2019 that's been in the South all it's life. Only had 17,000 miles when I got it in September, and had been garage kept the entire time. It's now got around 22K miles.

I have probably 16" of bleed hose line.
 

GTIfan99

Autocross Champion
Location
FL
Time to part out. 😪

Might be time to let a good shop look at it. A second set of eyes is good sometimes.
 

GTIfan99

Autocross Champion
Location
FL
I don't know if this will make you feel better or worse, but it took an entire season of ax, tracking, bleeding dozens of times, running the abs pump, and a front and rear pad change to finally dislodge the air I had in the system.
 

woodardhsd

Ready to race!
Location
NC
Car(s)
2020 GTI S DSG
Just thought I would update this thread.

TLDR version:
Dealer agreed to look it at again, diagnosed as bad booster, but didn't fix the problem. Dealer opened a tech line case without any resolution. Replaced the master cylinder myself, with little if any improvement. Found out engaging the parking brake makes brakes feel better for a minute.

Long Version:
While picking up my wife's Sportwagen from the dealer after they did a DSG fluid change, I got a chance to speak with the lead tech for a few minutes. Explained the situation with the brakes, and let him test out the brake pedal. Turns out they had another Rabbit waiting for an alternator so he fired it up and we compared the pedal feel. The other rabbit's brakes we're much better, almost exactly how I remembered my 2020's brakes feeling. He agreed there was a difference, and that they would schedule an appointment for a more thorough diagnosis. They would perform a bleed with their machine to make sure it wasn't air related and go from there. I did mention that I was about fed up and if they weren't going to do anything, I was going to replace the master cylinder myself. He said he had never seen a bad master cylinder in his 15 years, and that he thought it was probably booster related.
Brought it back over for that appointment, and sure enough, they did a bleed and found no air in the system. The tech's diagnosis was brake booster, so they ordered one. I told the service writer that didn't make sense, but that I would let them go through the process. The part finally came in and was replaced, but with no improvement in pedal feel. They admitted they didn't know what else to do, so they opened a tech line case and scheduled another time for me to bring it back. After another appointment, the dealer said that the tech line people could see nothing wrong with the data coming from the dealership. I asked how tech line was supposed to see something mechanical or hydraulic over a computer, but he didn't really have an answer. I went to pick up the car and tell the dealer to go to hell, but they told me that they were escalating my case up a level and that someone from tech line may have to come out and see the car. They said they would reach back out when they heard back from tech line. That was 3 weeks ago.
A week ago, I decided to just replace the master cylinder myself. There seemed to be a marginal improvement, but it's not perfect. I didn't have enough brake fluid to perform a thorough bleed, but it's at least at the same place it was before. I will give it another bleed this weekend to make sure there's no more air.
I did discover one interesting thing 2 days ago. Ended up parking in a different location for work and the space had a little downhill slope, so I engaged the parking brake. When I left after work, the brakes felt amazing, jut like my old car. However, after a few stops the effect wore off and the pedal travel increased again. I was able to re-produce that effect by engaging the parking brake each time I park or stop, but it always goes away after a while. Not sure what is happening here, unless the parking brake mechanism in the caliper wasn't self adjusting correctly and the piston was re-setting farther pack into the caliper bore than it should. I did reset the pistons back into their bore a few months ago in an effort to get all the air out. I'm not sure I understand this one.
 

victorofhavoc

Autocross Champion
Location
Kansas City
Just thought I would update this thread.

TLDR version:
Dealer agreed to look it at again, diagnosed as bad booster, but didn't fix the problem. Dealer opened a tech line case without any resolution. Replaced the master cylinder myself, with little if any improvement. Found out engaging the parking brake makes brakes feel better for a minute.

Long Version:
While picking up my wife's Sportwagen from the dealer after they did a DSG fluid change, I got a chance to speak with the lead tech for a few minutes. Explained the situation with the brakes, and let him test out the brake pedal. Turns out they had another Rabbit waiting for an alternator so he fired it up and we compared the pedal feel. The other rabbit's brakes we're much better, almost exactly how I remembered my 2020's brakes feeling. He agreed there was a difference, and that they would schedule an appointment for a more thorough diagnosis. They would perform a bleed with their machine to make sure it wasn't air related and go from there. I did mention that I was about fed up and if they weren't going to do anything, I was going to replace the master cylinder myself. He said he had never seen a bad master cylinder in his 15 years, and that he thought it was probably booster related.
Brought it back over for that appointment, and sure enough, they did a bleed and found no air in the system. The tech's diagnosis was brake booster, so they ordered one. I told the service writer that didn't make sense, but that I would let them go through the process. The part finally came in and was replaced, but with no improvement in pedal feel. They admitted they didn't know what else to do, so they opened a tech line case and scheduled another time for me to bring it back. After another appointment, the dealer said that the tech line people could see nothing wrong with the data coming from the dealership. I asked how tech line was supposed to see something mechanical or hydraulic over a computer, but he didn't really have an answer. I went to pick up the car and tell the dealer to go to hell, but they told me that they were escalating my case up a level and that someone from tech line may have to come out and see the car. They said they would reach back out when they heard back from tech line. That was 3 weeks ago.
A week ago, I decided to just replace the master cylinder myself. There seemed to be a marginal improvement, but it's not perfect. I didn't have enough brake fluid to perform a thorough bleed, but it's at least at the same place it was before. I will give it another bleed this weekend to make sure there's no more air.
I did discover one interesting thing 2 days ago. Ended up parking in a different location for work and the space had a little downhill slope, so I engaged the parking brake. When I left after work, the brakes felt amazing, jut like my old car. However, after a few stops the effect wore off and the pedal travel increased again. I was able to re-produce that effect by engaging the parking brake each time I park or stop, but it always goes away after a while. Not sure what is happening here, unless the parking brake mechanism in the caliper wasn't self adjusting correctly and the piston was re-setting farther pack into the caliper bore than it should. I did reset the pistons back into their bore a few months ago in an effort to get all the air out. I'm not sure I understand this one.
How are your rear brakes looking in terms of pad glaze and pad wear? Are they like record discs? Can you grab a caliper and measure the dtv on them?

When you pull the handbrake the rear ratchets down. As you do a few stops the rear lets go slightly. If you have enough wear or variation in the disc, I'm thinking it could wiggle the rear pads apart enough to notice longer pedal stroke to reengage the rotor. Handbrake would obviously ratchet it back down each time.
 

woodardhsd

Ready to race!
Location
NC
Car(s)
2020 GTI S DSG
Don't think I measured the discs, but the pads had around 7 or 8 mm of material left on them. I'm not getting any sort of weird vibrations or pulsing with the brakes, just a soft pedal.

I don't think the rotors are glazed, but I can't say for certain. I've attached photos of the rear brakes from my 2020 GTI, as well as my Rabbit. They look pretty much identical to me.
 

Attachments

  • 2020 GTI rear brakes.jpg
    2020 GTI rear brakes.jpg
    733 KB · Views: 48
  • 2019 GTI rear brakes.jpg
    2019 GTI rear brakes.jpg
    907.3 KB · Views: 32

victorofhavoc

Autocross Champion
Location
Kansas City
Don't think I measured the discs, but the pads had around 7 or 8 mm of material left on them. I'm not getting any sort of weird vibrations or pulsing with the brakes, just a soft pedal.

I don't think the rotors are glazed, but I can't say for certain. I've attached photos of the rear brakes from my 2020 GTI, as well as my Rabbit. They look pretty much identical to me.
The front brakes apply something like 80% of the pressure the rears do, so you wouldn't really feel a disc warp under light pressure unless you're familiar with the feeling and know what to look for. It's hard to tell from just looking at the disc, you really do have to measure it with a caliper at various points to determine if there's a lot of thickness variation.

There's a common track occurrence known as "pad knock" which is extremely prevalent on fixed calipers since they don't move and the disc will rock back and forth with G-force on track. It creates a soft pedal feel with a longer stroke and a common technique is to "brush" the brake lightly down the straights to be ready for braking at the end of the straight and into the corner.

Floating calipers resist pad knock more, but only if the slide pins are in good, greased condition and properly torqued. If the system is otherwise perfect, you're not getting major bubbles out when bleeding, and it goes away when you brush the brakes lightly before applying full pressure, it's not an internal brake system issue. Could be just one warped disc, could be two, could be heavy pad glaze on as little as one inside of a rotor or multiple. It can also occur if your disc isn't sitting exactly square against the hub. The little screw that holds the disc on should always thread in with ease and require virtually no torque to sit fully flush. You can dismount everything except for the rotor and then spin the rotor and look for any lateral play. There should be zero play overall.

Is everything oem with the knuckles, discs, hubs/bearings, lugbolts?
 

GTIfan99

Autocross Champion
Location
FL
Is it possible the abs system is messed up?
 

woodardhsd

Ready to race!
Location
NC
Car(s)
2020 GTI S DSG
The front brakes apply something like 80% of the pressure the rears do, so you wouldn't really feel a disc warp under light pressure unless you're familiar with the feeling and know what to look for. It's hard to tell from just looking at the disc, you really do have to measure it with a caliper at various points to determine if there's a lot of thickness variation.

Is everything oem with the knuckles, discs, hubs/bearings, lugbolts?

I attempted to measure the thickness with my amazon digital dial caliper and the results were within a few hundredths of a milimeter. However, I'm not sure I trust the caliper along with my measuring technique. I didn't feel like removing the rotors from the car to measure. I would think if it were enough to cause the pad-to-rotor gap to open up that I would be able to feel pulsing. I believe everything is 100% OEM and original. I got the car with around 17K on it so I can't be certain.

I don't know if this will make you feel better or worse, but it took an entire season of ax, tracking, bleeding dozens of times, running the abs pump, and a front and rear pad change to finally dislodge the air I had in the system.
I'm starting to think you may be right. After I noticed the deal with the e-brake, I bled just the rears 2 weekends ago and got a little more air out. After I got a brake fluid refill from FCP last week, (thank baby jesus for their lifetime replacement on fluids) I bled the entire system again last weekend. Got a little more air out of the rears, and a few tiny bubbles from the fronts. I also cracked the fittings going into the ABS module from the master cylinder just for the hell of it. That last bleed made a huge difference. The brakes feel good about 90% of the time now, compared to the 10% it was before. It might just need one more good bleed to get it all out.
 

woodardhsd

Ready to race!
Location
NC
Car(s)
2020 GTI S DSG
Well, that didn't last long. After the last bleed (5/17) , the brakes went from feeling great 90% of the time to around 50%. I bled the rears again (5/31) and got a little more air out of the RR but not the LR. Brakes felt great again for a while, but the sponginess has returned more often than not. I'm assuming that air has to be getting in the system somehow, but I have no idea how that would be possible since there are no leaks.
 

victorofhavoc

Autocross Champion
Location
Kansas City
Well, that didn't last long. After the last bleed (5/17) , the brakes went from feeling great 90% of the time to around 50%. I bled the rears again (5/31) and got a little more air out of the RR but not the LR. Brakes felt great again for a while, but the sponginess has returned more often than not. I'm assuming that air has to be getting in the system somehow, but I have no idea how that would be possible since there are no leaks.
In this case I would say it has to be one of the internal seals in the MC... Problem is you mentioned before this doesn't occur when the handbrake or rear brakes have been depressed. Is that still true?

If so the only points where air could be getting in might be one of the rear caliper seals...

You've checked rear brake wear, right? No pads at an angle or excessively worn?

Have you tried to go on a drive and then check temps on each of the rotors and calipers? Are the xds and other brake settings all factory set? All sensors reporting normal for abs, wheel speed, etc?
 

woodardhsd

Ready to race!
Location
NC
Car(s)
2020 GTI S DSG
In this case I would say it has to be one of the internal seals in the MC... Problem is you mentioned before this doesn't occur when the handbrake or rear brakes have been depressed. Is that still true?

If so the only points where air could be getting in might be one of the rear caliper seals...

You've checked rear brake wear, right? No pads at an angle or excessively worn?

Have you tried to go on a drive and then check temps on each of the rotors and calipers? Are the xds and other brake settings all factory set? All sensors reporting normal for abs, wheel speed, etc?
Suspected bad seals were the reason I replaced the MC back in early May. After replacing the MC, I disassembled as much as I could and the seals/pistons appeared in good shape. I have noticed that engaging the handbrake doesn't seem to have as much effect as it did when I first noticed that behavior.
Air getting in the rear calipers, especially the RR, is kinds what I'm leaning towards.
Rear pads are wearing evenly, and still have 6-7mm of thickness left on them.
I have not attempted to measure brake temps as I do not have a infrared thermometer to do so. Probably wouldn't hurt to try if this continues. I have not made any coding changes to the brakes/differential. I'm assuming all sensors are working correctly since I have no codes or errors.

Update:
After last post, I have just been driving it, no work has been done to the car. The brakes would be on the squishy side for a few days, then firm up for a few. They firmed up and felt good for few days before I was going out of town, but then the car sat for 4 days. I was worried I was going to come back and have mushy pedal again, but it was fine.
They slowly started getting mushy again earlier this week. One particular stop the pedal was really soft, and nearly went to the floor. When I did that I really pushed it hard towards the floor and held it while sitting at the traffic light. Not sure what happened there, but since then, the brakes have felt fantastic.
I did order new rear pads to install even though the original pads have plenty of life left. Was hoping (probably wishful thinking) that any residual air might be easier to bleed out if the pistons are farther up inside the bore.
 

victorofhavoc

Autocross Champion
Location
Kansas City
Suspected bad seals were the reason I replaced the MC back in early May. After replacing the MC, I disassembled as much as I could and the seals/pistons appeared in good shape. I have noticed that engaging the handbrake doesn't seem to have as much effect as it did when I first noticed that behavior.
Air getting in the rear calipers, especially the RR, is kinds what I'm leaning towards.
Rear pads are wearing evenly, and still have 6-7mm of thickness left on them.
I have not attempted to measure brake temps as I do not have a infrared thermometer to do so. Probably wouldn't hurt to try if this continues. I have not made any coding changes to the brakes/differential. I'm assuming all sensors are working correctly since I have no codes or errors.

Update:
After last post, I have just been driving it, no work has been done to the car. The brakes would be on the squishy side for a few days, then firm up for a few. They firmed up and felt good for few days before I was going out of town, but then the car sat for 4 days. I was worried I was going to come back and have mushy pedal again, but it was fine.
They slowly started getting mushy again earlier this week. One particular stop the pedal was really soft, and nearly went to the floor. When I did that I really pushed it hard towards the floor and held it while sitting at the traffic light. Not sure what happened there, but since then, the brakes have felt fantastic.
I did order new rear pads to install even though the original pads have plenty of life left. Was hoping (probably wishful thinking) that any residual air might be easier to bleed out if the pistons are farther up inside the bore.

Have you looked at the abs pump?
 

victorofhavoc

Autocross Champion
Location
Kansas City
I wouldn't even know where to begin with the ABS pump. I've certainly considered the possibility that it was the problem. Something is definitely letting air into the system cause the brakes are back to being as shitty as ever.
If you have a good mechanic around they can bench bleed it. You can do it yourself, but you'll have to properly pull and plug the pump before adding charge to it to make it cycle.

Otherwise there's also a way to run the pump using vagcom while it's in the car. The dealer can obviously do this part easily.

If it still seems like it could be related to the rear, you could always pull the hard lines for the rear at the pump and plug it, then go for some short drives with only the front brakes and see how that changes things. Obviously be safe, at your own risk, Yada Yada.
 
Top