Low vacuum unless brake pedal pressed

Ljwelch

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Joined
Jan 27, 2025
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Nh
TDI
2001 jetta alh
I have a new issue on my 2001 tdi this all started because i had a boost leak so ive been looking for a possible vacuum leak i hooked a vacuum gauge to the nipple in the middle of the hose going from the vac pump to the brake booster it only gets 10inhg however if i push the brake pedal in it increases to about 25ish inhg if i take the end of the hose that goes into the brake booster out and put my finger over the end it gets 25inhg so im assuming its a brake booster issue anyone have any ideas of what this is and if its fixable or i just need a new booster along with that the hose from the vacuum pump to the booster is brand new so no cracks in that that could be causing it open to anything worth trying
 

Ljwelch

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Jan 27, 2025
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Nh
TDI
2001 jetta alh
Yeah, do you remember that little black tube going into the booster having a seal issue at the booster?

That’s my recollection, and sometimes the tube itself cracks…
The hose is brand new and holds vacuum if i take it off and cover the hole with my finger however to be safe i put some black silicone around the contact surface still have same issue though
 

Lug_Nut

TDIClub Enthusiast, Pre-Forum Veteran Member
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Jun 20, 1998
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Sterling, MA. USA
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2015 GSW 6M in S trim the other oil burners: 1967 two stroke Sonett 1988 Bolens DGT1700
Is the old rubber grommet into which that new hose installed allowing leaks around that new hose or maybe at the grommet to booster chamber?
Take that brand new hose loose AT IT'S OTHER END and leave the booster end in the booster. Put your finger over that open end and apply your vacuum pump and gauge.
 

Ljwelch

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Jan 27, 2025
Location
Nh
TDI
2001 jetta alh
Is the old rubber grommet into which that new hose installed allowing leaks around that new hose or maybe at the grommet to booster chamber?
Take that brand new hose loose AT IT'S OTHER END and leave the booster end in the booster. Put your finger over that open end and apply your vacuum pump and gauge.
Are you saying to unhook it from the pump and pressurize it to see if it holds vacuum i think i understood that right
 

jimbote

Certified Volkswagen Nut
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Jul 10, 2006
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spiral arm, milky way (aka central NC)
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Tacoma 4x4 converted to TDI
Bad booster. Boosters work by having equal vacuum on both sides of the diaphragm. When you push the pedal, the vacuum is released in a modulated amount from the pedal side of the booster. This gives a vacuum bias to the master cylinder side of the booster. Also when you hit the brakes, it not only releases vacuum from the pedal side, it isolates the two sides from each other. So basically, the faulty internal control valve is bleeding off vacuum from both sides constantly, until you hit the pedal, in which case the master side is isolated, and vacuum can rebuild.
 
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Ljwelch

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Jan 27, 2025
Location
Nh
TDI
2001 jetta alh
Bad booster. Boosters work by having equal vacuum on both sides of the diaphragm. When you push the pedal, the vacuum is released in a modulated amount from the pedal side of the booster. This gives a vacuum bias to the master cylinder side of the booster. Also when you hit the brakes, it not only releases vacuum from the pedal side, it isolates the two sides from each other. So basically, the faulty internal control valve is bleeding off vacuum from both sides constantly, until you hit the pedal, in which case the master side is isolated, and vacuum can rebuild.
Could this be the cause of my boost leak too then because its not getting vacuum to the system i can only build about 2 psi
 

P2B

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Jan 11, 2006
Location
Toronto & Muskoka, Canada
TDI
2002 Jetta, 2003 Jetta, 2003 Jetta Wagon
Lack of vacuum will prevent the turbo from building boost when the ECU requests it. A boost leak is loss of boost between turbo and intake
 

Ljwelch

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Joined
Jan 27, 2025
Location
Nh
TDI
2001 jetta alh
Lack of vacuum will prevent the turbo from building boost when the ECU requests it. A boost leak is loss of boost between turbo and intake
Okay so thats probably my issue cause it builds boost but only 2psi and its not like its leaking its just itll only build that much and it basically stop
 

Ljwelch

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Jan 27, 2025
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Nh
TDI
2001 jetta alh
Update i tried smoke testing the brake booster and i saw no leaks is this something i would see leaking out if i had a vacuum leak in the booster
 

Ljwelch

Member
Joined
Jan 27, 2025
Location
Nh
TDI
2001 jetta alh
Honestly, you've already done all the testing to narrow it down to the booster. If you have to go one step further, use a hand pump to apply vacuum to the booster. No vacuum build=bad booster.
Im just nervous its not gonna fix my problem but i suppose if its bad its probably a good thing to change thank you
 

eddieleephd

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Joined
May 27, 2012
Location
Battle Ground, Wa
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2002 jetta Wagon
That is a grommet the plastic piece goes into, so, both inside and outside the grommet must seal… usually the leak comes from the inside seal…
 

Ljwelch

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Joined
Jan 27, 2025
Location
Nh
TDI
2001 jetta alh
That is a grommet the plastic piece goes into, so, both inside and outside the grommet must seal… usually the leak comes from the inside seal…
I tried to do a quick fix to see if it would work by putting gasket maker around those spots no luck
 

DS4465

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Jul 5, 2024
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Grand Duchy of Luxembourg.
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2001 Audi A3 1.9 ALH, 2004 E46 M47TU Coupe, 2002 E39 M57 Touring.
The hose is brand new and holds vacuum if i take it off and cover the hole with my finger
The problem was troubleshot and correctly diagnosed by 2 people above.
Sometimes trusting your findings is the most difficult part when you try to fix your car.

Is there a way i can test for a leak in the booster
With everything connected as normal, start the engine and run it for 1-2 minutes, give it some RPM to build some vacuum pressure in the booster. Then brake firmly and while holding the brake pedal down, shut down the engine. You should be able to keep the brake pedal down with the same pressure on the pedal for +/- 30 seconds. What you will feel, is the brake master cylinder pushing your foot/brake pedal immediately back, because the brake booster leaks internally and cannot hold assist pressure.

Just my 2c. _
 
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