Using brakes causes vacuum loss?

I've been searching all over and I can't seem to find anyone else who has posted the same thing so hopefully I didn't miss it while searching. The problem that I am having is when I push the brake pedal, my vacuum holds steady vacuum (27in) but when I let off the pedal, my vacuum falls to 20 in. It slowly rebuilds after that. If I pump the brakes i can easily pull to 0 vacuum. My brakes work fine if I haven't used them in say 30 seconds, but if I'm going slowly in traffic and am using brakes often, after the first couple of times, my pedal becomes hard as if my booster isn't working.

Sounds like you have a vacuum leak somewhere. Good news is, the BHW has the simplest vacuum system of any TDI sold here. So it is easy to check the entire system for the most part. The only issue is the N75 valve and vacuum reservoir are in a tight spot down low at the front of the engine. But if the engine runs OK, and no MIL is on, you at least know adequate vacuum is getting to the turbo actuator... but if it is bad it may be all that the system can supply to keep it pulled in.

But first give the system a visual check, starting with the tube that runs along the front side of the battery under the cowl cover to the booster, and through the firewall grommet and over to the vacuum pump on the engine.

So far in troubleshooting, I have checked the hoses to the booster and they all are good. Then, to check to eliminate the rest of the vacuum system, i capped off the line to the n75 and tried again with the engine idling and i still am getting the same results. My first thoughts are that the booster is leaking. But I don't want to just throw parts at it either.

Well if you have a bad booster, you have the very first one I have ever heard of on a B5.5 Passat. But it is possible. See if you can contrive a way to use a hand vacuum pump on just the booster servo itself, and pump it up.

If you want to make a road trip you can have the booster from a crash car I have. Still in it, but it should still be good.

I can pull vacuum with the mighty vac and it holds. Once i pump them and release 1 time i can only pull 10 inches with my mighty vac but it will hold that steady

Would a weak vacuum pump do this? Would it still be able to pull 27 or so inches of vacuum even if its weak?

I am not sure what is "normal", but obviously ~28 at sea level (or whatever the numbers work out to be) is ideal. But you have to consider recovery time. You may have a weak pump. You can take and push a little motor oil into it (the correct type, as it will be pushed into the engine) and see if it improves. I fixed a few like that, that must have had some sticky flapper seal things inside.

Ok I'll try that next. Thanks

KnoxWilson said:

The problem that I am having is when I push the brake pedal, my vacuum holds steady vacuum (27in) but when I let off the pedal, my vacuum falls to 20 in. It slowly rebuilds after that. If I pump the brakes i can easily pull to 0 vacuum.

Click to expand...

That's exactly how the booster works, when you press the pedal you're 'bleeding' off the vacuum on one side of the booster diaphragm. Release the pedal and the vacuum pump has to pump that side down again.

http://www.agcoauto.com/content/news/p2_articleid/129

KnoxWilson said:

if I'm going slowly in traffic and am using brakes often, after the first couple of times, my pedal becomes hard as if my booster isn't working.

Click to expand...

^^^^ this is the part that's "not normal", at least from my experience.

Yes, vacuum will be quickly consumed, and the vacuum bottle will help a bit, but the pump should be able to keep up with brake usage in normal stop-and-go traffic... at least in my experience.

I had a BHW tandem pump fail and the first sign was stiffening brake pedal. It got to the point that the pedal was stiff all the time in a short time span, and the tandem pump could only pull 5" of vacuum. I swapped the tandem pump and haven't dissected the old one yet to see what failed.

Most of them fail with fuel leaking but not mine. It had about 265k miles on it with no fuel leaks.

Also, check the vacuum part of the tandem pump for a loose nipple. I just replaced mine, and the nipple was moving around in the housing causing a leak.

I have the same issue on my B6 A4 TDi Quattro Sport Avant... (PD 130+)


I eventually traced the fault to a poorly fitting/broken vacuum hose attached to the pump itself...

The one that is a fairly solid plastic hose that feeds the brake servo... then splits off to the turbo etc...


As a temp repair until I can get a replacement pump as it looks like it is the vacuum output side that has broken, I cleaned the area completely with degreaser, and then applied silicone sealant liberally around the area of the break, started the engine and let it tick over whilst working the silicone into all the nooks and crannies, whilst the vacuum helped to draw it in to the broken/damaged area as well.

Switched off and left overnight and so far not a leak and no loss of brake assistance nor turbo pressure and spool up....


Just waiting for the new pump to arrive.....


I think Windex has pinpointed the area above...


FF

Had the wife call this morning that "the brakes aren't working" on our '05 Passat wagon. I haven't been home to check it out yet, but I'm guessing the power assist is gone so she thinks the brakes aren't working.

I will check the vacuum lines and the "T" tonight, but I'm wondering if the loss of vacuum could also be responsible for the heat suddenly not working in the car? We recently had the AC hard lines replaced, and frankly I don't know if we've use the heat since, but when driving my mother-in-law to the airport over the weekend, the heat was not functioning. Engine is running at normal operating temps (well as much as I can tell with the dash gauge) and the AC appears to cool, but no heat.


Herb

Quest4TDI said:

Had the wife call this morning that "the brakes aren't working" on our '05 Passat wagon. I haven't been home to check it out yet, but I'm guessing the power assist is gone so she thinks the brakes aren't working.

I will check the vacuum lines and the "T" tonight, but I'm wondering if the loss of vacuum could also be responsible for the heat suddenly not working in the car? We recently had the AC hard lines replaced, and frankly I don't know if we've use the heat since, but when driving my mother-in-law to the airport over the weekend, the heat was not functioning. Engine is running at normal operating temps (well as much as I can tell with the dash gauge) and the AC appears to cool, but no heat.


Herb

Click to expand...

Well I 'think' I got my brake issue squared away. I checked the vacuum lines at the "T" behind the engine. One of them was fairly crispy. I replaced it (luckily I had some spare 4mm silicone vacuum hose laying around) and then proceeded to test the brakes. Initially the car still had the problem, but after cautiously driving it for a minute or two, the vacuum built up and the brakes operated normally again.

Because I only 'think' I fixed the brake issue, I'm going to do some more testing over the next day or two before I left my wife drive it again.

BTW, even though I don't have large hands, getting to the "T" was a nightmare, let alone removing a stuck on vacuum hose.


Herb