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Discussion Starter #1
Howdy to all
I just tested my brake booster with a Myte-Vac and found that the booster will not hold any vacuum at all. From my previous automotive experience this would be an indication that the booster needs to either be rebuilt or replaced. So my questions to this forum are:

Do the Saab brake boosters hold a vacuum? Is not holding a vacuum an indication of part failure?

Can the boster be rebuilt? Or is it a throw away?

Would anybody out there be willing to send cash to Oregon?:D

Thanks.

Ron
 

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The brake booster has to hold vacuum, otherwise you have no power assist to the brakes. Usually that is easy to determine on a cold start, just with your foot on the brake pedal. The assist is about 4:1, can't miss that, unless you have a wooden leg... :cheesy:

Testing the vacuum servo can be a little tricky.
 

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Discussion Starter #3
Yup, that's what I was expecting but before I spend a box full of cash on a booster I thought it would be worth asking.

Is tehre a link somewhere to walk me through testing the servo?

No woden legs here, but there is that glass eye thing.;)

Now back to my last question about sending cash to Oregon:D

Ron
 

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I don't remember anyone posting the procedure, and I have not done it myself on this car.

So, you are certain the brake booster does not work, meaning, there is no power assist to the brakes? And when you start the car with one foot firmly on the brake pedal, it does not sink even a little when vacuum builds in the intake?

I do know it is not that simple. There is a one way valve (check valve mid-way in the vacuum line from the intake manifolt to the servo, the vacuum line from the intake to the servo is usually pretty hardened by now, and so on.

Where did you disconnect the vacuum connection betweent he manifold and the brake servo?
 

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Discussion Starter #5
PMI said:
So, you are certain the brake booster does not work, meaning, there is no power assist to the brakes? And when you start the car with one foot firmly on the brake pedal, it does not sink even a little when vacuum builds in the intake?

I do know it is not that simple. There is a one way valve (check valve mid-way in the vacuum line from the intake manifolt to the servo, the vacuum line from the intake to the servo is usually pretty hardened by now, and so on.

Where did you disconnect the vacuum connection betweent he manifold and the brake servo?
I'm pretty sure, I disconnected the line directly from the face of the booster so the only thing that I'm testing is the booster itself. And yes, all of the lines are hardened to the point of being brittle. I thinking that if I go after a new booster i will also replace all of the lines from the manifold to through the check alve and to the manifold.

As I'm writting this I'm thinking that the next test could be to insert a "T" and check running vacuum. Maybe I'll try that before I drop a bunch of cash.

Ron
 

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oregonxv said:
As I'm writting this I'm thinking that the next test could be to insert a "T" and check running vacuum.
Sounds like a good idea. It is not unheard of for the diaphram to fail on any car, but it does not seem like a common failure here.

I've seen posts about the brake booster being suspect, and testing it the oldfashioned way, but can't remember anyone actually having to replace the booster itself on this car.

http://www.saabcentral.com/forums/showthread.php?t=55652
 

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Discussion Starter #7
Still Chasing

Well, I'm still chasing brake problems. I didn't jump into replacing the booster and master cylinder, primarily because I just couldn't find anyplace where there is any record of these cars having the problem.
I did flush the system with vacuum and new fluid, which did help. I have been trying for some time to chase down the reason that causes the Vacuum assist pump to run continuosly and still don't have a solution for that one. I can pull the relay out of the panel on the left hand side of the engine compartment near the fire wall and it goes off. And then when I replace the relay and drive the car for any distance it returns to the continuous running symptom.
Arggggg.
Any help in diagnosing the contiuous rinning of the vacuum assist pump would be greatly appreciated.

Ron
 

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A good booster will hold it's vacuum for days if not weeks; the test is simple, drive then pull up, switch off and leave the vehicle for an hour or two, then try the brakes, you'll feel the assist of the vacuum, then after 2-3 tries the pedal gets harder as you lose the vacuum stored. Most common issues with the brake boost servo is not the unit but its non-return valve.
 

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Discussion Starter #10
PMI said:
Which relay, exactly?

On my car the 'F' section has two relays, one for Fog Lights and the other for Vac. Pump.
 

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oregonxv said:
On my car the 'F' section has two relays, one for Fog Lights and the other for Vac. Pump.
The vacuum pump is nothing to do with the brakes, that is for the emission control. the brakes use engine vacuum.
 

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Discussion Starter #12
ragtopcav said:
The vacuum pump is nothing to do with the brakes, that is for the emission control. the brakes use engine vacuum.
If that's true then I have a couple questions.
1) why is is plumbed ONLY to the brake system?
2) Why is the replacement pump part listed on eEuroparts.com as a "Brake Assist Vacuum Pump"?
 

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oregonxv said:
If that's true then I have a couple questions.
1) why is is plumbed ONLY to the brake system?
2) Why is the replacement pump part listed on eEuroparts.com as a "Brake Assist Vacuum Pump"?
That is a bit baffling; I can walk up my yard a take a gander at a 1996 900set, a 1997 Talladega set, a 1997 9000 2.3T and 2 other less relevant Saabs and all of them have a crude plastic hose, with a non-return valve, from the brake booster to the inlet manifold. Usually there is a vacuum pump for cars with some sort of EGR type system and it is situated in the from wing on your driver's side.
 

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Discussion Starter #14 (Edited)
ragtopcav said:
That is a bit baffling; I can walk up my yard a take a gander at a 1996 900set, a 1997 Talladega set, a 1997 9000 2.3T and 2 other less relevant Saabs and all of them have a crude plastic hose, with a non-return valve, from the brake booster to the inlet manifold. Usually there is a vacuum pump for cars with some sort of EGR type system and it is situated in the from wing on your driver's side.
Yup, that's where the pump is located, directly below the battery behind the front bumper. My understanding is that the "Air Injection Pump" is located at the same place but on the 2.3 Litre configurations bound for Sweden and U.S. Spec. Miine is the 2.0 Turbo Automatic and of course U.S. Spec, so as I understand the theory, they added the vacuum pump as an assist to the lower vacuum levels resulting from the rest of the engine design.
I am very curious about the non-return valve though. I don't have any documentation that would tell me which one it is or what an appropriate test would be. Kind of searching in the dark.
 

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Discussion Starter #15
Here's one more small piece of information that might help figure this out. With the car turned off but with the pump still running I can pull the small vacuum line from the switch at the fire wall and connect a gauge. The pump is pulling a full 15 inchs of vacuum at that point. The manual that I have states that the pump should turn on at 0.35 BAR abd back off at 0.40 BAR. From my information that would convert to 10.3 to 11.8 inches, I'm thinking more and more about the non-return valve and the electrical controls that should be turning the pump off.
I'm also thinking of new signatures,

Lost in the dark
Searching through the fog
Frustrated and confused

I'll stick with the polite ones for the list.
 

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Two different pumps, the SAI pump and the vacuum assist pump are different systems.

I don't have the vacuum assist on my car (manual trans), and I did not even know there was a relay, which is why I asked. This makes sense now. :roll:

(Ragtop will have to run out and buy another Saab, to complete his collection... :cheesy: )

A check valve is shown in both setups, the simple one on my car, and the more complex one with the vacuum assist. The plastic tube comes off the intake manifold, on the side close to the servo. About as thick as a finger. The check (nonreturn) valve is inline in the tube/pipe.

This is just a guess, but diagrams show a pressure monitor on the cars with the vacuum assist. If the vacuum at the brake servo is insufficient, the pressure monitor may be what turns on the relay that operates the vacuum assist pump.

The pressure monitor looks like it has the same type of rubber vacuum hose (same p/n) as the rest of the vacuum hoses which dry out, crack and leak on every car, too (!)

Good luck w. the new signature ;)
 

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oregonxv said:
...The manual that I have states that the pump should turn on at 0.35 BAR and back off at 0.40 BAR.
Any chance you can test that with the hand pump? Plug the fuse back in, let the pump start, draw a vacuum using the hand pump and a piece of vacuum hose, and see if the monitor turns off the pump at 0.40.

You could also put in a T, and see if the vacuum ever gets to a decent level at that point with the pump running.

Hey, this testing is much more fun when I am not the one freezing his butt off (as usual), :cheesy:
 

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PMI said:
(Ragtop will have to run out and buy another Saab, to complete his collection... :cheesy: )
PMSL :lol:

From what you've posted that makes sense, the configuration for vacuum sensing and assistance is rather like what I would expect to find on a diesel car.
 

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PMI said:
So, you are certain the brake booster does not work, meaning, there is no power assist to the brakes? And when you start the car with one foot firmly on the brake pedal, it does not sink even a little when vacuum builds in the intake?
Just want to thank you PMI for confirming in my head what I thought was happening in my car.

I switch the intake manifold to that of a non turbo; Suddenly, it takes a long time for vac assist to connect with the brakes. I'd assumed it was because of change in volume in the intake manifold, but was not certain.

Unnerving some mornings, as I forget and then have really rugged brakes for about a minute!

-bny
 

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Discussion Starter #20
PMI said:
Two different pumps, the SAI pump and the vacuum assist pump are different systems.

I don't have the vacuum assist on my car (manual trans), and I did not even know there was a relay, which is why I asked. This makes sense now. :roll:

(Ragtop will have to run out and buy another Saab, to complete his collection... :cheesy: )

A check valve is shown in both setups, the simple one on my car, and the more complex one with the vacuum assist. The plastic tube comes off the intake manifold, on the side close to the servo. About as thick as a finger. The check (nonreturn) valve is inline in the tube/pipe.

This is just a guess, but diagrams show a pressure monitor on the cars with the vacuum assist. If the vacuum at the brake servo is insufficient, the pressure monitor may be what turns on the relay that operates the vacuum assist pump.

The pressure monitor looks like it has the same type of rubber vacuum hose (same p/n) as the rest of the vacuum hoses which dry out, crack and leak on every car, too (!)

Good luck w. the new signature ;)
Ok, this is beginning to make some sense now. The vacuum assist connects on the Booster side of the non-return valve. Between the non-return connection and the assist pump is a "T" where the rubber hose connects and goes to the vacuum/electric switch that I mentioned earlier. When I tested how much vacuum was building from the pump earlier it was this hose that I used so I'm confident that it is in good order.
So if I'm seeing this correctly, the switch on the firewall doesn't seem to be controlling the pump. One of the tests that I have done is to disconnect the signal wire from this switch while the pump was running, the pump don't stop so I thought at the time that the switch wasn't controlling the pump. Now I'm not so sure. So now I have a couple more questions,
1) From the schematics that you have, is the switch on the fire wall supposed to control the pump function?
2) Am I correct that the function of the non-return valve is that it is basically a one way vacuum valve - in other words "One Way" towards the booster? Which defines how to test it.
3) If I disconnect the booster and plug the line and the pump turns off then all else is ok and I have isolated a booster failure - correct?

Thanks for all the help.

Additional signatures under consideration.
Saab stories R-Us
Just another Western Oregon Saab Story
Learning English - The Svedish Vay:D

Ron
 
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