# Bad brake master cylinder?



## SconnieGTI (Jun 10, 2012)

I've tried searching and I can't find anything that's helping so my last resort is to post a thread.

Mk4 1.8T GTI. 158k.

So a few days ago I had to slam the brakes and wound up putting the pedal almost to the floor. Yesterday after about an hour of highway driving, I exited and as I went to stop, I noticed the pedal was tight, but I wasn't really stopping. So I started pumping the brakes and got stopped in time, but barely. Didn't notice anymore issues after that and drove the same distance back home, plus some more city driving with no issues so I just chalked it up to possibly having snow packed in the calipers/pads. (Huge snow storm 2 days ago and I like to play.)

The car sat overnight and all day until about 730pm, and I noticed the same issue. The pedal was tight but I wasn't really stopping. So I kept pumping the brakes for a couple blocks and then they started acting right again.

Tonight I checked the brake lines from the calipers to as far as I could follow them and didn't notice any fluid or gunk build up. In fact all brake lines look to be in really good condition. And the res is full.

This leads me to believe it has to be the brake master cylinder but I'd like some other opinions. If it is the MC.. does anyone know of a DIY for replacing it? I searched and could only find clutch MC DIY's.

Any and all help is super appreciated.

Sent from my Man Cannon via Tapatalk 2


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## KG18t (Aug 9, 2006)

A shot master cylinder would result in pedal going to the floor. If the pedal is stiff and you have to stand on it to stop - you may have a vacuum issue - either the hose to the booster or the booster itself. Your 'tight but have to pump it' description seems a little contradictory.


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## SconnieGTI (Jun 10, 2012)

The issue I noticed was that after an hour of highway driving (not using the brakes) and also after sitting all night, the next time I went to use the brakes, the pedal felt tight, but I wasn't stopping like I should. Like I said, I thought maybe I just had snow or ice built up between the pad and rotor since we had just had a really slushy/wet snow storm 2 nights before, and I was out bombing and sliding around. But after both instances, the brakes worked fine after using them again a couple times. Which I thought was due to a leak in the lines, thus pumping/using them was building the pressure back up. But it's possible I'm mistakin'..

Is there any other way to diagnose a bad booster? Or do you basically just replace it every so often?


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## SconnieGTI (Jun 10, 2012)

After more investigation and reading about the break booster going out.. when I push the brake pedal down (car off) I can hear air being pushed out from under the pedal somewhere. After about 6-7 pumps, the brakes tightened up (as in the pedal only moved an inch or so) and the noise went away.

Then I started the car and noticed the brakes were soft again. Could this be the brake booster, and not the MC?

Videos added:

Maybe these will help someone who knows more about this issue.. maybe there isn't an issue at all and I'm overreacting. I don't know lol.


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## KG18t (Aug 9, 2006)

Still trying to figure out what you're saying... stop saying tight, that doesn't describe anything useful. What you described using the pedal after the car is off - a few pumps and then the pedal gets harder - this is normal. When the car is running, it builds up vacuum in the booster which assists the pedal press. After it's off, the vacuum is sustained for a while, using the brake drains the boost... so to speak.

You said "to the floor" in your first post. Did the pedal actually go all the way to the floor? Or did you just have to stand on it?

With the car running, press on the brake pedal with enough force for a normal stop. It should stop, and hold the same force against your foot, if it loosens up and begins to fall to the floor, you have a bad MC.

If you press the pedal after starting the car and it goes to the floor, but stiffens up after you pump it a few times, you should bleed the brakes, and try again.


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## SconnieGTI (Jun 10, 2012)

KG18t said:


> Still trying to figure out what you're saying... stop saying tight, that doesn't describe anything useful. What you described using the pedal after the car is off - a few pumps and then the pedal gets harder - this is normal. When the car is running, it builds up vacuum in the booster which assists the pedal press. After it's off, the vacuum is sustained for a while, using the brake drains the boost... so to speak.
> 
> You said "to the floor" in your first post. Did the pedal actually go all the way to the floor? Or did you just have to stand on it?
> 
> ...


I assumed I probably wasn't coming off as clear as I intended to. My apologies. Not literally to the floor, but yes I did have to stand on it.

At the bolded, that IS what happens. So then that means my MC is fine. Good to hear. :thumbup:

I will try bleeding the brakes when I get time and report back. Thanks for your help. :wave:


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## radlynx (Jan 4, 2007)

when braking, do you have to press it hard to stop the car or not? 



SconnieGTI said:


> I assumed I probably wasn't coming off as clear as I intended to. My apologies. Not literally to the floor, but yes I did have to stand on it.
> 
> At the bolded, that IS what happens. So then that means my MC is fine. Good to hear. :thumbup:
> 
> I will try bleeding the brakes when I get time and report back. Thanks for your help. :wave:


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