I have an interesting issue with my friend's quantum. The brakes work fine if you stop quickly, but on long hills if you use the brakes gently, the vacuum assist goes away and you need to push very hard. Kinda scary at times. I'm under the impression that it is either the freeplay adjustment or a problem with the vacuum booster. I'd rather not mess with the freeplay adjustment if it won't help, but don't want to spend the $170 for a vac booster if it is just a freeplay adjustment. Anyone have thoughts or opinions?
Andrew
Loosing Vacuum Assist
Moderator: Fatmobile
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Vincent Waldon
- Turbo Charger
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In my experience freeplay issues are fairly constant.. ie don't come and go depending on frequency of use.
Frequency of use problems are almost always an issue with the boost system pressure... not enough pressure in the system over sustained usage. Either not enough pressure is being generated fast enough to replenish the system (ie vac pump issue) or pressure is being dissipatedly quickly with repeated use (vacuum leak to the booster or the booster itself). There's a check valve in between the supply and demand that can fail as well... not 100% sure it's on the diesels however.
The Bentley says:
"On cars without ABS, test the vacuum booster by first pumping the brake pedal a few times with the engine off, and then holding the pedal down. When the engine is started, the pedal should fall slightly. The booster diaphragm rarely fails, but leaks in the vacuum line or a faulty check valve will decrease the vacuum assist. Check the valve by removing it from the booster vacuum line and blowing through it. Air should pass through in the direction of the arrow on the check valve, but not through the opposite way. On cars with diesel engines, vacuum booster problems may be caused by a faulty vacuum pump. If brake fluid is leaking into the booster from the master cylinder, the master cylinder is defective and should be replaced."
Have you got a vacuum gauge lying around ? If so I'd be inclined to hook it up as close to the booster as possible and see what the pressures are like... if it's a slow dissipation then I'd look for a hose leak... if it drops radically when the brakes are activated and doesn't recover that sounds like a check valve or a leaking booster.
Never used a vacuum gauge on this circuit but I have one... happy to experiment on my (working!) brakes if you need a comparison... all in the name of VW science !!
Too bad you can't use soapy water to look for a vacuum leak.
Vince
Frequency of use problems are almost always an issue with the boost system pressure... not enough pressure in the system over sustained usage. Either not enough pressure is being generated fast enough to replenish the system (ie vac pump issue) or pressure is being dissipatedly quickly with repeated use (vacuum leak to the booster or the booster itself). There's a check valve in between the supply and demand that can fail as well... not 100% sure it's on the diesels however.
The Bentley says:
"On cars without ABS, test the vacuum booster by first pumping the brake pedal a few times with the engine off, and then holding the pedal down. When the engine is started, the pedal should fall slightly. The booster diaphragm rarely fails, but leaks in the vacuum line or a faulty check valve will decrease the vacuum assist. Check the valve by removing it from the booster vacuum line and blowing through it. Air should pass through in the direction of the arrow on the check valve, but not through the opposite way. On cars with diesel engines, vacuum booster problems may be caused by a faulty vacuum pump. If brake fluid is leaking into the booster from the master cylinder, the master cylinder is defective and should be replaced."
Have you got a vacuum gauge lying around ? If so I'd be inclined to hook it up as close to the booster as possible and see what the pressures are like... if it's a slow dissipation then I'd look for a hose leak... if it drops radically when the brakes are activated and doesn't recover that sounds like a check valve or a leaking booster.
Never used a vacuum gauge on this circuit but I have one... happy to experiment on my (working!) brakes if you need a comparison... all in the name of VW science !!
Too bad you can't use soapy water to look for a vacuum leak.
Vince
Vince
Edmonton, Alberta, Canada
2001 silver TDI Jetta Malone Stage 1.5 , 2001 blue TDI Jetta SBIII 216s Malone Stage 3
1970 Bay Window bus
Gone but not forgotten: 1969/1971 Beetles, 1969/1974 Westies, 1979 Rabbit, 1986 TD Jetta, 1992 gas Jetta, 1994 TD Jetta
Here's a small collection of HOW-TOs
Edmonton, Alberta, Canada
2001 silver TDI Jetta Malone Stage 1.5 , 2001 blue TDI Jetta SBIII 216s Malone Stage 3
1970 Bay Window bus
Gone but not forgotten: 1969/1971 Beetles, 1969/1974 Westies, 1979 Rabbit, 1986 TD Jetta, 1992 gas Jetta, 1994 TD Jetta
Here's a small collection of HOW-TOs
Maybe I didn't word it right. If you give decent pedal pressure (even for an extended period of time) the vacuum assist does not go away. It is only if you do very light braking, in which case the vacuum assist dissipates very quickly. In other words, my impression is that there is a vacuum leak in the system when the pedal is pushed slightly, but not when it is pushed more solidly. With regard to vac supply, there is no issue there. It's been a while since I tested it with a gauge, but I remember thinking it had max vacuum right off idle.
Andrew
Andrew
I had a similar problem last summer. When the engine reached operating temperature the car would do about the same thing. Only the brake pedal would have a bunch of play in it after which made the amount of push I could give it to be very minimal creating the unsafe braking condition.
I tested a new vacuum booster, check valve and my master cylinder was a month old... Turns out it was the brake booster.
Might help.
-Matt
I tested a new vacuum booster, check valve and my master cylinder was a month old... Turns out it was the brake booster.
Might help.
-Matt
1990 TD Jetta - 490,000Km
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Fatmobile
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brakes
Sometimes when pushing the brake lightly, it's because you are going slowly.
Could the problem be RPM related.
My Golf has weak brakes at low RPMs but if I've been driving at higher RPMs the brakes work great.
It just doesn't build up enough vacuum at lower RPMs.
Could the problem be RPM related.
My Golf has weak brakes at low RPMs but if I've been driving at higher RPMs the brakes work great.
It just doesn't build up enough vacuum at lower RPMs.
'91 Golf gasser converted to a 12mm pump, M-TDI.
'84 1.6TD Rabbit with a VNT-15 turbo, still setup to run on vegetable oil.
'84 GTI with 1.7TD pistons and intercooled.
2003 TDI wagon
2000 TDI Jetta.
'84 1.6TD Rabbit with a VNT-15 turbo, still setup to run on vegetable oil.
'84 GTI with 1.7TD pistons and intercooled.
2003 TDI wagon
2000 TDI Jetta.
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Vincent Waldon
- Turbo Charger
- Posts: 1148
- Joined: Sat Dec 16, 2006 3:05 pm
- Location: Edmonton, Alberta, Canada
Sadly it still sounds like the booster to me... diaphragm that leaks more with mild deflection than when you press firmly on the petal.
Vince
Vince
Vince
Edmonton, Alberta, Canada
2001 silver TDI Jetta Malone Stage 1.5 , 2001 blue TDI Jetta SBIII 216s Malone Stage 3
1970 Bay Window bus
Gone but not forgotten: 1969/1971 Beetles, 1969/1974 Westies, 1979 Rabbit, 1986 TD Jetta, 1992 gas Jetta, 1994 TD Jetta
Here's a small collection of HOW-TOs
Edmonton, Alberta, Canada
2001 silver TDI Jetta Malone Stage 1.5 , 2001 blue TDI Jetta SBIII 216s Malone Stage 3
1970 Bay Window bus
Gone but not forgotten: 1969/1971 Beetles, 1969/1974 Westies, 1979 Rabbit, 1986 TD Jetta, 1992 gas Jetta, 1994 TD Jetta
Here's a small collection of HOW-TOs