# Warning: Lack of Power Brake Assist on MK4 with high boost



## TyrolSport (Mar 4, 1999)

I've been dealing with a strange issue for 2 years now, and it began to get worse at an AutoX yesterday. At very random times on track, when transitioning from heavy boost to hard braking, I get NO power assist and a heavy pull to the right. It happened twice yesterday in the same place on the AutoX course, and happens at the same spot at Watkins Glen. I will be changing the one way valve to the brake booster, but was wondering if anyone else had any suggestions or has seen this before....


----------



## reflexgti (Dec 19, 2000)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (tyrolkid)*

Are you lifting the inside rear tire while braking? I've noticed on the mk4's that the brake pedal gets very hard when this happens. I would guess it's due to the ABS pump trying to prevent lockup on the inside rear. 
Next time, try disabling the ABS to test out the theory. Good luck.


----------



## TyrolSport (Mar 4, 1999)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (reflexgti)*


_Quote, originally posted by *reflexgti* »_Are you lifting the inside rear tire while braking? I've noticed on the mk4's that the brake pedal gets very hard when this happens. I would guess it's due to the ABS pump trying to prevent lockup on the inside rear. 
Next time, try disabling the ABS to test out the theory. Good luck.

I know what you are describing but this is different. It even happens when let's say I'm at full boost for a few seconds, then I go to half throttle for a slalom(no braking). At the next braking zone, I'll have a hard pedal....


----------



## reflexgti (Dec 19, 2000)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (tyrolkid)*

Hmm, that's very odd. Has this always happened or just something that's popped up recently?


----------



## TyrolSport (Mar 4, 1999)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (reflexgti)*


_Quote, originally posted by *reflexgti* »_Hmm, that's very odd. Has this always happened or just something that's popped up recently?

It's something that has happened very randomly since I started tracking the car. It happened at Watkins Glen and at the AutoX, but never at Lime Rock. Makes no sense......


----------



## phatvw (Aug 29, 2001)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (tyrolkid)*


_Quote, originally posted by *tyrolkid* »_It's something that has happened very randomly since I started tracking the car. It happened at Watkins Glen and at the AutoX, but never at Lime Rock. Makes no sense......

Perhaps you could hookup a laptop+vagcom in the car and do some datalogging but I'm not sure what exactly you'd want to log. Maybe ask in the VAG-COM forum?
If you have a boost controller or different ECU programs, perhaps you could try limiting the boost/power output to a lower value to further correlate the issue.


----------



## Mohudsolo (Feb 25, 2004)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (phatvw)*

Given any thought to simply running a small electric vacuum pump and eliminate the connection to the engine? I wonder if the inline valve is being overpowered by the higher than stock boost and pressurizing the system instead of keeping it at vacuum.


----------



## corrado94 (Sep 22, 2003)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (tyrolkid)*

Mike maybe a faulty check valve on the vac brake booster??? something in the vacuem system ???. Dont want that poping up at the end of the straightway at limerock .







Bob.G


----------



## TyrolSport (Mar 4, 1999)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (corrado94)*


_Quote, originally posted by *corrado94* »_ Mike maybe a faulty check valve on the vac brake booster??? something in the vacuem system ???. Dont want that poping up at the end of the straightway at limerock .








Bob.G

I'm going to be replacing the brake booster checkvalve as a precaution....


----------



## Banditt007 (Oct 23, 2002)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (tyrolkid)*

bump, what was the outcome?


----------



## TyrolSport (Mar 4, 1999)

*Re: Warning: Lack of Power Brake Assist on MK4 with high boost (Banditt007)*

hose is still in my trunk.....getting to it this week before my first event of the season.


----------



## bettonracing (Feb 28, 2005)

If u feel like fabbing up something u can hook the booster hose to pre-turbo. (never in boost condition. Always vacuum)

P.S. I know I'm like 5 yrs late on this post but for information purposes...)


----------



## TyrolSport (Mar 4, 1999)

*Re: (bettonracing)*

Update:
We installed an electric vacuum pump in the system and tested it at Watkins Glen over the last couple of days. The previous issue was both a hard pedal after extended boosting, accompanied by a hard pull to the right. The electric vac pump solved the hard pedal, but we are still experiencing the hard pull at random times. The hard pull has been isolated as the right rear wheel locking up under braking. The pull was so severe that we spun the car into the gravel trap while braking in a straight line. The car came around with no warning after 4 sessions without issue. Attempts at modulating the brake pedal during the lockup are useless. The wheel just stays locked even when the pedal is pumped








We were emailed a year ago by someone on here reporting similar issues....if that person is reading this, can you please email us again to see if you have made any progress. Thanks!


----------



## [email protected] (Jul 11, 2007)

Just a thought, but it's entirely possible that you had no problems with the power assist system in it's natural state (the wheel locking up is another matter). It's just that in an auto-x or road course, you're hard on the brakes, exhuasting the built up vacuum needed to facilitate the assist, and then right back on the gas, and you haven't given the vacuum booster a chance to catch back up before you're on the brakes again. 


_Modified by [email protected] at 10:34 AM 9-19-2007_


----------



## TyrolSport (Mar 4, 1999)

*Re: ([email protected])*


_Quote, originally posted by *[email protected]* »_Just a thought, but it's entirely possible that you had no problems with the power assist system in it's natural state (the wheel locking up is another matter). It's just that in an auto-x or road course, you're hard on the brakes, exhuasting the built up vacuum needed to facilitate the assist, and then right back on the gas, and you haven't given the vacuum booster a chance to catch back up before you're on the brakes again. 

That is correct, and that is why the aftermarket vacuum pump solved the lack of power assist. It was strange how both conditions would always appear simultaneously. Now that the power assist is fixed, we are left with the right rear wheel locking randomly. Different calipers, pad compounds and fluids have not changed the situation. I'm guessing it is something in the ABS module, but there are no fault codes


----------



## phatvw (Aug 29, 2001)

*Re: (tyrolkid)*

Hey I wonder if its possible to reproduce the wheel-locking in a safe environment like an open parking lot? Perhaps log all 4 wheel speeds and see if you see a pattern.
Maybe its just that right-rear wheel speed ABS sensor that is faulty?
Also, I think there is a gauge you can hook up in place of the bleeder screw to see what the actual fluid pressure is when you stand on the brakes. You might want to get one of those and see if the left vs right pressure is equal. Probably won't notice any difference when the engine bay is cold. SO heat things up a bit








Or perhaps the mechanical components in the ABS controller itself are on their way out, or the metal bits inside have warped or something. You could try popping in a new MC/ABS unit. Rather expensive test, but worth it to not have a spin-out like that again.


_Modified by phatvw at 11:30 AM 9-19-2007_


----------



## jamesb (Mar 14, 2001)

*Re: (phatvw)*

What is the corner balence of your car? I had to add a decent amount of weight to my right rear to get both the golf and jetta to stop locking even in a straight line stop.


----------



## 20AE-2306 (Jan 21, 2006)

*Re: (jamesb)*

I have this problem. To reproduce it, try hard braking a couple of times while you keep the engine out of vacuum and the pedal will get totally solid. Then when you let off the gas, the brake pedal will soften again... this can also happen at other times when performance driving...
Maybe a vacuum resevour would help. Maybe one could tap the DV vac resevour and/or replace it with something with more capacity.
Sometimes I also get this error on my cluster that says "STOP!!" and something about brake fluid. Probably the level sensor thinks the level is low because the fluid is slushing around. Not sure how to fix, maybe overfilling the brake fluid (sounds like a bad idea though).


----------



## david_594 (May 16, 2004)

All MK4 TDI's have a vacuum resevoir. If you guys are looking for one, I might be worth scrounging one from a wrecked TDI.


----------



## TyrolSport (Mar 4, 1999)

*Re: (david_594)*

We added an electric vacuum pump to the car last month and tested it at Watkins Glen. The pump solved the issue with the pedal becoming solid. However, it did not stop the right rear wheel from locking up and sending me into the gravel trap.


----------



## 20AE-2306 (Jan 21, 2006)

*Re: (tyrolkid)*

what kind of vacuum pump did you use? does it run all the time or does it know when there is no vacuum in a can of some kind?


----------



## Super 180s (Nov 14, 2007)

*Re: (tyrolkid)*


_Quote, originally posted by *tyrolkid* »_We added an electric vacuum pump to the car last month and tested it at Watkins Glen. The pump solved the issue with the pedal becoming solid. However, it did not stop the right rear wheel from locking up and sending me into the gravel trap.









Caliper rebuild time?


----------



## TyrolSport (Mar 4, 1999)

*Re: (20AE-2306)*


_Quote, originally posted by *20AE-2306* »_what kind of vacuum pump did you use? does it run all the time or does it know when there is no vacuum in a can of some kind?









We will be posting a technical how-to shortly on this. It has a built in pressure sensor so that it turns on only when needed.


----------



## TyrolSport (Mar 4, 1999)

*Re: (Super 180s)*


_Quote, originally posted by *Super 180s* »_
Caliper rebuild time?

Tried rebuilding as well as new calipers. No luck.


----------



## Super 180s (Nov 14, 2007)

*Re: (tyrolkid)*

Intermittent fault with one of the pistons in the ABS pump?


----------



## TyrolSport (Mar 4, 1999)

*Re: (Super 180s)*


_Quote, originally posted by *Super 180s* »_Intermittent fault with one of the pistons in the ABS pump?

That is the current hypothesis, but unfortunately, I have to put myself in potentially lethal situations in order to test whether or not the repair works. I can't afford to crash or injure myself if swapping the ABS controller doesn't solve the problem. Given the fact that there are multiple people with this issue(we have been contacted offline), we're beginning to think that it might be in the actual software of the ABS module.....


----------



## Super 180s (Nov 14, 2007)

Don't spose there's any deserted airfields anywhere near you where you have loads of runoff?
Sent you an IM.


----------



## 20AE-2306 (Jan 21, 2006)

*Re: (tyrolkid)*


_Quote, originally posted by *tyrolkid* »_
That is the current hypothesis, but unfortunately, I have to put myself in potentially lethal situations in order to test whether or not the repair works. I can't afford to crash or injure myself if swapping the ABS controller doesn't solve the problem. Given the fact that there are multiple people with this issue(we have been contacted offline), we're beginning to think that it might be in the actual software of the ABS module.....

That would be cool, if you've actually been able to push the car so far outside of it's design parameters as to run into problems like this. I guess one solution might be to turn off the ABS, I doubt it would have enough time to be useful on auto-x.
I push my car hard enough for the brake pedal to get stiff and for the cluster to freak out with a brake fluid fault telling me to "PULL OVER". The latter may be some air in the brake lines or something because I definitely have exactly the right amount of fluid... I don't know if that's normal while performing evasive maneuvers involving a lot of cornering. I haven't pinned down exactly when it happens because I'm usually busy having accidentally triggered the wipers and the high-beams...


----------



## ianacole (Aug 11, 2003)

I ran into the same problem of hard pedal and no braking in autocross situations, but never on the track. I tried to solve this with the Stainless Steel Brake Company electric vacuum pump (this one: http://www.ssbrakes.com/produc...year=), but the pump died and I didn't like the extra weight. We're looking into using the Passat (?) electric vacuum assist pump, but with the car becoming more track oriented and less autocross oriented I'm not sure that we're going to worry about it. I'd like to hear what you did to overcome this issue.
I have not had any issues with wheels locking up like you've experienced, which sounds like you're leaning to ABS programming as being the culprit. Depending on the results of my off a couple of weeks ago, we may be ditching the ABS system anyways.
Good luck figuring it out, and please keep us up to date on what you find.


----------



## phatvw (Aug 29, 2001)

*Re: (ianacole)*

I think if you ditch ABS, I think you need to add a front/rear proportioning valve. Otherwise, its going to be difficult to get the brake bias just right. Plus having the adjustment for rainy race conditions and different tire setups would be good.
I wonder what brake controllers come on the Beettle cup cars?


----------



## ianacole (Aug 11, 2003)

*Re: (phatvw)*


_Quote, originally posted by *phatvw* »_I think if you ditch ABS, I think you need to add a front/rear proportioning valve. Otherwise, its going to be difficult to get the brake bias just right. Plus having the adjustment for rainy race conditions and different tire setups would be good.
I wonder what brake controllers come on the Beettle cup cars?

Definitely would use a brake proportioning valve, which managing the bias is another argument for ditching the ABS system.


----------



## 20AE-2306 (Jan 21, 2006)

*Re: (ianacole)*

I think if you have a vac pump, you may as well run all your vacuum solenoids off it. Idle stability is greatly effected by this, especially it seems, on the 1.8t. The DV, maybe PCV (with a filter probably), definitely SAI and Evap. This would also reduce the chance of having boost leaks...
In order to keep it from running constantly, I would think that adding some more vac resevours would help. Ones similar to the blue balls in the passenger fender should be capable of storing quite a bit of vacuum. The larger they were, the less often the pump would need to run. You would need a sensor to check for a low vacuum condition, perhaps -19hg minimum, -21hg max.
Would be an all-around winner as long as the pump was reliable.


----------



## Daisyjane (Nov 23, 2007)

*Re: (20AE-2306)*

My Mazda Rockstead was having a problem with its brake lately and I don't like it!







Good thing i had my portable radio in my car that cools my mind. However it's inevitible for to me to worry for this might cause an accident. In fact I've been checking more information in sirius to handle this problem.


----------



## 20AE-2306 (Jan 21, 2006)

*Re: (Daisyjane)*

Is that spam or did you really mean to say that?


----------



## ianacole (Aug 11, 2003)

*Re: (Daisyjane)*

Spam...and report


----------

