Brakes Locking - 1.8t

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Brakes Locking - 1.8t

Post by ZiF_k »

Evening everyone.

Having some issues with the brakes on the Polo of recent.
Some background information, not sure if this will help to point me in the right direction.
Ecu was replaced with an Mk4 gti ecu some time ago due to the lambda drivers no longer working. Since then I've had a traction light on which was caused by not having the abs pump coded to the ecu from what I was told. Brake pedal was fine for a while. After a while the pedal got spongy. Brakes worked still but took some time to stop the car. Eventually I came to the conclusion that the booster was an issue. This got changed out for a booster from an Ibiza Cupra Tdi. The booster from this and the polo looked identical. The only real issue I had was the brake light would not go off but that got sorted. Also in the process I fitted a set of AP calipers from a Cupra. These were serviced with new seals and stuff before they were fitted so I know these are fine.

The issue I now have is that the brakes lock up after a short drive. Once this happens I bleed the calipers manually and carry on driving. I was told that the abs pump could be an issue or the booster may be an issue since it came off a tdi. Is there anyway to test either of these? I can't seem to bleed the abs pump from abs because of the pump not being coded to the ecu. Someone also mentioned to me that tdi uses a vacuum pump for the brake booster and the polo uses intake pressure which could also cause an issue. Is there anything I can look at or test to maybe isolate the cause of this? I will scan this week to see if what errors come up.

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Re: Brakes Locking - 1.8t

Post by missioner »

You must still have air in the system or a faulty master cylinder.

I'm inclined to go with a new master cylinder. I had a car once that did something similar to yours and the master sorted out the issue. Going by your history with brakes I feel this may have been the root of your issues from the beginning.
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

I've replaced the master cylinder last year as well the slave. I bought the brake booster used with a master cylinder which I was told was working. Maybe I should swap that out and see if there is any difference. I've bled the brakes multiple times and I'm pretty sure there is no air in the system.

I've spoken to Andrew (Pynoxim) and he told me to check if the speed works to confirm if the abs pump is working. Speedo works so that might not be an issue.

I was also told that the push rod length may need to be adjusted. I'm not sure if this can be done on the Cupra/Polo brake booster.

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Re: Brakes Locking - 1.8t

Post by missioner »

ZiF_k wrote:I've replaced the master cylinder last year as well the slave. I bought the brake booster used with a master cylinder which I was told was working. Maybe I should swap that out and see if there is any difference. I've bled the brakes multiple times and I'm pretty sure there is no air in the system.

I've spoken to Andrew (Pynoxim) and he told me to check if the speed works to confirm if the abs pump is working. Speedo works so that might not be an issue.

I was also told that the push rod length may need to be adjusted. I'm not sure if this can be done on the Cupra/Polo brake booster.

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Push rod could also help.

How much free play do you have on the pedal?
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

missioner wrote:
ZiF_k wrote:I've replaced the master cylinder last year as well the slave. I bought the brake booster used with a master cylinder which I was told was working. Maybe I should swap that out and see if there is any difference. I've bled the brakes multiple times and I'm pretty sure there is no air in the system.

I've spoken to Andrew (Pynoxim) and he told me to check if the speed works to confirm if the abs pump is working. Speedo works so that might not be an issue.

I was also told that the push rod length may need to be adjusted. I'm not sure if this can be done on the Cupra/Polo brake booster.

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Push rod could also help.

How much free play do you have on the pedal?
This I would have to check. I know I had to 'make a plan' to get the brake light switch to work.

Would you know if the pushrod can be adjusted?

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Re: Brakes Locking - 1.8t

Post by missioner »

ZiF_k wrote:
missioner wrote:
ZiF_k wrote:I've replaced the master cylinder last year as well the slave. I bought the brake booster used with a master cylinder which I was told was working. Maybe I should swap that out and see if there is any difference. I've bled the brakes multiple times and I'm pretty sure there is no air in the system.

I've spoken to Andrew (Pynoxim) and he told me to check if the speed works to confirm if the abs pump is working. Speedo works so that might not be an issue.

I was also told that the push rod length may need to be adjusted. I'm not sure if this can be done on the Cupra/Polo brake booster.

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Push rod could also help.

How much free play do you have on the pedal?
This I would have to check. I know I had to 'make a plan' to get the brake light switch to work.

Would you know if the pushrod can be adjusted?

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Not off hand, I would have to look at the setup to know.

Come to think of it your brake light problem could very well be due to the same issue.
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

missioner wrote:
ZiF_k wrote:
missioner wrote:
ZiF_k wrote:I've replaced the master cylinder last year as well the slave. I bought the brake booster used with a master cylinder which I was told was working. Maybe I should swap that out and see if there is any difference. I've bled the brakes multiple times and I'm pretty sure there is no air in the system.

I've spoken to Andrew (Pynoxim) and he told me to check if the speed works to confirm if the abs pump is working. Speedo works so that might not be an issue.

I was also told that the push rod length may need to be adjusted. I'm not sure if this can be done on the Cupra/Polo brake booster.

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Push rod could also help.

How much free play do you have on the pedal?
This I would have to check. I know I had to 'make a plan' to get the brake light switch to work.

Would you know if the pushrod can be adjusted?

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Not off hand, I would have to look at the setup to know.

Come to think of it your brake light problem could very well be due to the same issue.
Maybe I can take some pictures of the old booster for reference. They looked identical when I checked them side by side.

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Re: Brakes Locking - 1.8t

Post by ZiF_k »

Some pics of the booster. Part numbers are identical. Didn't manage to get to vcds this evening. Will hopefully manage to get a scan done tomorrow. ImageImageImageImage

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Re: Brakes Locking - 1.8t

Post by missioner »

See all the rust on the outside where the M/c bolts on, that's usually a sign that it's toast.

You said that you got another master with the booster so hopefully it's ok and not leaking internally. One of the ways to check is to hold down the pedal as long as you can and see if it drops down over time.

The pushrod for the master doesn't have any adjustment by the looks of it. The pedal side on many cars has some sort of adjustable rod with a lock nut, this one seems like it doesn't.

I will ask a friend what he thinks about your issue.
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Re: Brakes Locking - 1.8t

Post by ZiF_k »


missioner wrote:See all the rust on the outside where the M/c bolts on, that's usually a sign that it's toast.

You said that you got another master with the booster so hopefully it's ok and not leaking internally. One of the ways to check is to hold down the pedal as long as you can and see if it drops down over time.

The pushrod for the master doesn't have any adjustment by the looks of it. The pedal side on many cars has some sort of adjustable rod with a lock nut, this one seems like it doesn't.

I will ask a friend what he thinks about your issue.
I saw all the rust and also thought it looked a bit worse for wear. The replacement was in much better condition. I've also got my oem one which I replaced for no reason previously.

I will do the pedal test tomorrow and report back on that.

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Re: Brakes Locking - 1.8t

Post by ZiF_k »

Did the test on the master this morning and the pedal stayed more or less where should be. Didn't drop after the initial press and hold. Pedal still felt quite firm.

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Re: Brakes Locking - 1.8t

Post by missioner »

How much free play does the pedal have?

At the rest position the pedal must have some movement between the pedal and the push rod as well as some clearance between the pushrod and the booster/master.

If there is no free play, the compensation port on the master cylinders gets closed by the plungers of each brake circuit. The result is the brake fluid has no where to go when the fluid heats up and expands. This will cause the brakes to begin to bind and then lock up when you are driving. The pedal will get hard and lock up the brakes at the slightest press. The wheels get hot after a short drive and other symptoms like ABS activation and so on.

Check if the pedal has a stopper bolt on the seat facing side, loosen it off and see what happens.

4 to 8 mm of play on the brake pedal is what I would expect most times, measured at the bottom of the pedal.
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

missioner wrote:How much free play does the pedal have?

At the rest position the pedal must have some movement between the pedal and the push rod as well as some clearance between the pushrod and the booster/master.

If there is no free play, the compensation port on the master cylinders gets closed by the plungers of each brake circuit. The result is the brake fluid has no where to go when the fluid heats up and expands. This will cause the brakes to begin to bind and then lock up when you are driving. The pedal will get hard and lock up the brakes at the slightest press. The wheels get hot after a short drive and other symptoms like ABS activation and so on.

Check if the pedal has a stopper bolt on the seat facing side, loosen it off and see what happens.

4 to 8 mm of play on the brake pedal is what I would expect most times, measured at the bottom of the pedal.
I will check this tomorrow and report back. Thanks missioner.

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Re: Brakes Locking - 1.8t

Post by PanTheR11 »

Have you made any new discoveries Zif ?
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

PanTheR11 wrote:Have you made any new discoveries Zif ?
Haven't had time between work and weddings this past week. This weekend is also out. Hopefully I get a chance sometime soon. So far I think I'm leaning towards the booster being an issue as well. I hear a hissing noise near the pedal which is quite loud. I want to take the car for a drive and when it locks up, remove the vacuum pipe from the booster to check if it releases just to confirm.

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Re: Brakes Locking - 1.8t

Post by PanTheR11 »

ZiF_k wrote: Fri Apr 09, 2021 8:31 am
PanTheR11 wrote:Have you made any new discoveries Zif ?
Haven't had time between work and weddings this past week. This weekend is also out. Hopefully I get a chance sometime soon. So far I think I'm leaning towards the booster being an issue as well. I hear a hissing noise near the pedal which is quite loud. I want to take the car for a drive and when it locks up, remove the vacuum pipe from the booster to check if it releases just to confirm.

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Is the hissing sound not your alarm snake you installed the other day :troll:

Sounds to me like one massive vacuum leak there :bang:
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

So, brakes locked up again before even leaving the driveway. Removed the vacuum hose to the booster and the brakes released. So booster is coming off this week. Guy I got it from will swap it out for me no issues.

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Re: Brakes Locking - 1.8t

Post by PanTheR11 »

ZiF_k wrote: Sat Apr 10, 2021 5:01 pm So, brakes locked up again before even leaving the driveway. Removed the vacuum hose to the booster and the brakes released. So booster is coming off this week. Guy I got it from will swap it out for me no issues.

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Hope that solves your issues bru
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Re: Brakes Locking - 1.8t

Post by PapaJo »

Did you by any chance checked the non return vacuum unit in the booster line. Ensure that it is only sucking towards the engine/intake side and closes offwhen positive pressure comes from the engine/intake side.

When you pull the hose from booster did it sound as if the booster was under vacuum that released?
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Re: Brakes Locking - 1.8t

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PapaJo wrote: Sat Apr 10, 2021 9:23 pm Did you by any chance checked the non return vacuum unit in the booster line. Ensure that it is only sucking towards the engine/intake side and closes offwhen positive pressure comes from the engine/intake side.

When you pull the hose from booster did it sound as if the booster was under vacuum that released?
Great suggestion imho, the booster might be fine.
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Re: Brakes Locking - 1.8t

Post by ZiF_k »


PapaJo wrote:Did you by any chance checked the non return vacuum unit in the booster line. Ensure that it is only sucking towards the engine/intake side and closes offwhen positive pressure comes from the engine/intake side.

When you pull the hose from booster did it sound as if the booster was under vacuum that released?
I did double check the one way. Only allows air one way. So that's fine. But I was able to move the car once that vacuum line was removed.

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Re: Brakes Locking - 1.8t

Post by ZiF_k »

Another update. Changed the booster with a working booster which looked in much better condition than the previous one. Everything is identical to the stock one which was removed. Part numbers, measured sizes as well and all is exact. Front brakes are still locked. I'm now stumped. Both the boosters couldn't have been the same issue. Would a master cylinder cause the brakes to still lock up? ImageImage

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Re: Brakes Locking - 1.8t

Post by missioner »

Master cylinder is the only thing left that can
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Re: Brakes Locking - 1.8t

Post by ZiF_k »

missioner wrote:Master cylinder is the only thing left that can
I was also leaning towards this. Don't really see anything else other than the abs pump that might be faulty but that doesn't throw any codes.

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Re: Brakes Locking - 1.8t

Post by pyro-zn »

Would be easier to strip and sell for parts 🤦🏽‍♂️

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