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VW MKIV-A4 TDIs (VE and PD) This is a general discussion about A4/MkIV Jetta (99.5-~2005), Golf(99.5-2006), and New Beetle(98-2006). Both VE and PD engines are covered here.

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Old June 26th, 2016, 04:36   #1
ymz
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Fuel Economy: 5.3 l/100 km (at 115 km/h)
Default Brake Booster or Master Cylinder?

Is there a quick-and-dirty way to tell which has gone bad?

Yuri
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Old June 26th, 2016, 04:49   #2
jpdeuce
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What's the problem? Have you bled all the fluid out first and turned it all over with good/new fluid?
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Old June 26th, 2016, 05:40   #3
ymz
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I'm a couple of hours away from the car... only 2nd hand report that pushing down hard on the brake pedal isn't stopping the car... the fluid has NOT run low... (car has over 530,000 miles on it...)

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Old June 26th, 2016, 06:30   #4
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Moisture or bubbles in the fluid would likely give the same result, so it's hard to tell. At that mileage, who knows though if the MC/booster is original.

Has the fluid been flushed in the last 100k or so? That's the easiest and quickest to do, so I would start there. Don't use the pump method for bleeding though, so you don't introduce stuff into the lines that doesn't ned to be there. Maybe use a different color fluid so you can tell it's all the way flushed.
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Old June 26th, 2016, 06:37   #5
ymz
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Yes, fluid has been changed often enough... original MC and booster... I was just wondering if there's an easy way to tell other than having a local shop throw parts at the problem hoping that one of them would fix it... (If I'm able to go there, I'd check the vacuum hose going to the booster first, but, as there are no turbo issues, I suspect that the vacuum system's at least functioning...)
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Old June 26th, 2016, 08:45   #6
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Pull the vacuum line off from the booster and hook up a mity vac. It will take a while to build vacuum since the brake booster is so large, but it should be holding vacuum and not leaking out air. You would likely also be getting a hissing noise near the brake pedal if it were leaking vacuum.
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Old June 26th, 2016, 19:21   #7
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Is the pedal hard to press if so It could be the booster or it could be a vacuum leak between the vacuum pump and the booster. Check the plastic hose there known to split. Plenty of posts on it with pictures.
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Old June 26th, 2016, 19:34   #8
ymz
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Just spoke to my better half... it appears that once in a (rare) while, the pedal goes to the floor with no braking action... following which, everything's normal... until just last Friday... when things went South in a hurry...

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Old June 27th, 2016, 21:40   #9
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Quote:
Originally Posted by Hyde7278 View Post
Is the pedal hard to press if so It could be the booster or it could be a vacuum leak between the vacuum pump and the booster. Check the plastic hose there known to split. Plenty of posts on it with pictures.
+1
Just had this issue with those same symptoms.
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Old June 28th, 2016, 14:04   #10
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I had that issue right after new years. Just remember that if the master cyl is leaking into the booster it will eat the booster up. Replace them both while its apart.
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Old December 3rd, 2017, 06:22   #11
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Anyone ever figure out what their problem was? Having the exact same issues with my wife's 2001. I've ruled out the vacuum pump itself, also the hose to the booster. Next step is trying to find a mityvac and test the booster itself. The brakes work like S***. But once you hold the pedal down for a few seconds they lock right on. Also on very rare occasion the pedal will go right to the floor with no brakes at all, as described above
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Old December 3rd, 2017, 09:23   #12
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Quote:
Originally Posted by VRTn View Post
Anyone ever figure out what their problem was? Having the exact same issues with my wife's 2001. I've ruled out the vacuum pump itself, also the hose to the booster. Next step is trying to find a mityvac and test the booster itself. The brakes work like S***. But once you hold the pedal down for a few seconds they lock right on. Also on very rare occasion the pedal will go right to the floor with no brakes at all, as described above
Any work done recently?
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Old December 3rd, 2017, 11:06   #13
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If the pedal goes "to the floor" it isn't the booster. Either the seals in the master are bypassing or there's a leak in the system.

A leak will cause loss of fluid, bad seals on the master probably won't but might if the secondary seal that contains the fluid is bad too. If it is the fluid will go into the booster.

So...

Pedal to the floor, no loss of fluid indicates a bad master.

Pedal to the floor with fluid loss, look for a leak. If you find one fix it. If you don't the master is bad and the booster has fluid in it.

The delay then sudden lock up seems to implicate the booster but the pedal dropping is another thing. Seems like both the booster and master are bad.
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Old December 3rd, 2017, 15:50   #14
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Hold off on throwing money at it. My daily driver was doing the same thing. It would scare the crap out of you and then it work again. There is a fitting on the back of the vacuum pump that gets loose and causes a vacuum leak if the hose is moved. The nipple actually had come loose on mine and all it took was a little RTV once we got it cleaned up so it would stick. The fix has worked for over 9 months now. I almost pulled the trigger on a master cylinder and booster. Basically .50 cents worth of glue fixed it.
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Old December 3rd, 2017, 16:01   #15
VRTn
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No major work done recently, I put new brakes on it all around about 3 months ago and bled the whole system but it was doing it even before that. The pedal is nice and high and firm but just feels like the brakes are manual for the first 3 or 4 seconds of pushing the pedal then all of a sudden WHAM full braking power (which I've read several different cases of this happening but it usually always turns out to be the hard plastic vacuum line is cracked). I've replaced my hard line with rubber fuel injection line and clamped it with FI clamps. Checked the one way valve as well it is functioning properly, tapped the vacuum pump tight with a hammer and sealed it, replaced the rubber on the booster/main vacuum line and also replaced the vac storage bottle or whatever it's called, with the one from my parts car. But the odd time here and there (not very often, like once every few weeks) you push the pedal and it sinks down a ways, then you let it go and push it again and it's nice and high. I'm borrowing a hand vacuum pump from a friend and I'm going to test it and see if the booster leaks off. I've also heard you can do a smoke test on them as well. I don't think it has anything to do with the master because the fluid level hasn't went down and I don't have any leaks.... Very strange
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