924Board.org

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

I thought I fixed my brake issues by changing the vacuum lines. I drove the car and it seems that when the RPMs are up (above ~1000 or so) the brake booster works, but once the clutch is in and the RPMs drop to idle, then it is as if the vacuum must drop not allowing the booster to work. I will check my vacuum pressure to see if there is a drop or what. What could be causing the loss of vacuum? When replacing the vacuum lines per the sticker on the fender, it only had one line going to the "manifold vacuum limiter", but I have two (3/8") and one (1/8") lines connecting to the limiter. Is this right? Does anyone have a pic of their vacuum limiter?
Thanks,
_________________
Rob
'80 931 (sold)
'80 924 NA (sold)
Former Omaha Crew

Paul · Posted: Wed Aug 03, 2005 11:03 am Post subject:

There was a very recent thread on the vacuum limiter. Could you post a picture of your underhood sticker?

BTW vacuum is high at idle and low while accelerating. Is your check valve working? Is your booster air tight?

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

Paul · Posted: Wed Aug 03, 2005 12:25 pm Post subject:

The booster has a large rubber membrane that can crack and leak air.

After you shut off the car, remove the check valve from the booster and listen for air to rush in.

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Rob: if you can't get them posted, send them to me and I'll post them on my photobucket acct. Or go to photobucket.com and sign up for a free account.

That check valve seems like a good place to start looking. There should be a way to make sure that's working.
_________________
'80 924 n/a SOLD

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

Here is the picture of the vacuum diagram:

_________________
Rob
'80 931 (sold)
'80 924 NA (sold)
Former Omaha Crew

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

The thing I don't understand is when driving then stepping on the brake, the booster works, then when RPMs drop, the booster doesn't work. If the diaphragm is cracked, would it still work like it does?
_________________
Rob
'80 931 (sold)
'80 924 NA (sold)
Former Omaha Crew

Paul · Posted: Wed Aug 03, 2005 1:15 pm Post subject:

I suppose, I'm thinking maximum vacuum occurs when using engine braking, so maybe a leaking booster is the answer.

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

It seems as though the booster works only some of the time so i don't think that it leaks much, but I will check.
_________________
Rob
'80 931 (sold)
'80 924 NA (sold)
Former Omaha Crew

Smoothie · Posted: Wed Aug 03, 2005 2:36 pm Post subject:

The vacuum limiter should have 3 lines as you described - a small one sourced from the intake manifold (downstream of the throttle valves), a larger one attached upstream of the throttle valves and another large one attached downstream of the throttle valves.

You really need to put a vacuum gauge on it to see what's happening and when.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

My vacuum lines are exactly as seen in Smoothie's picture (see link)
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=341f&.dnm=f1db.jpg&.src=ph

I will check with a vacuum gauge in the next couple of days to see what is going on.
_________________
Rob
'80 931 (sold)
'80 924 NA (sold)
Former Omaha Crew

Smoothie · Posted: Thu Aug 04, 2005 3:57 am Post subject:

This won't have anything to do with your brake boost problem, but after seeing the '80 NA vacuum sticker, obviously your vacuum switch should be in-line with the upper distributor line and not as shown in that diagram I uploaded. -May not amount to a very big hill o' beans, but since it's shown that way on your sticker, might as well get it right.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

isuras2 · Joined: 17 Jun 2005 Posts: 458 Location: Phoenix, AZ

I have about 15 inches of vacuum at the booster connection. It did fluctuate lower (to ~10) with the cold engine. When I cracked the throttle, the vacuum decreased momentarily, then increased to around 25 or higher depending on how much throttle. Once I put a new grommet on the check valve/booster connection, the brakes seemed to work WITH the booster! I well go for a spin tomorrow to test everything as I did have a rear right brake dragging which might call for adjustment.

What should the inches vacuum be at idle? If the rear brakes aren't adjusted right, could that affect the booster at all?
_________________
Rob
'80 931 (sold)
'80 924 NA (sold)
Former Omaha Crew