Inconsistent and spongy brake pedel feel.

[woodardhsd]

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]

Time to part out.

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

 

[GTIfan99]

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]

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]

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]

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.

 

[victorofhavoc]

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]

Is it possible the abs system is messed up?