C900 brake booster diagnosis (vacuum servo assist)

There are three tests to accurately diagnose the condition of your brake booster.

Booster function (vacuum assist) - with engine off - depress brake pedal once or twice firmly and hold, start engine. Pedal should sink towards floor 2-3 inches. If it doesn't booster is faulty.

Air tightness check - with engine off - pump brake pedal several times. With each subsequent pump pedal travel should decrease. If it doesn't brake booster is faulty.

Air tightness under load - with engine on - depress brake pedal once firmly and hold. Turn engine off whilst still holding brake pedal. Pedal should not rise or sink for 45 seconds. If it does brake booster is faulty.

If your booster fails any of these 3 tests it is faulty and needs to be replaced.

Replacing the booster takes about 2 hours and is fairly easy. Remanufactured boosters range from $120-$180.

Thanks for this! I think I am going to move this one to the tutorials section. If you want to write any more informational stuff, go right ahead!!

Replacing brake booster - Difficulty 2/5 - Time required - 2 1/2 hours

- Remove center console if applicable, remove ashtray and 4 holdings screws, there is 1 10mm bolt that holds ashtray housing
- Remove knee board via 2x 10mm bolts in fender wells going into cabin (below the dash bolts)
- Locate brake booster inside cabin (it's difficult to access), remove 4x nuts that hold the booster to the firewall
- Remove the clevis pin arrangement which holds the booster pushrod to the brake pedal arm

In engine bay
- Remove 2 17mm nuts holding brake master cylinder to brake booster
- Pull brake master cylinder away from booster and off the two studs that fix it to the booster, you can move it with the brake lines still attached they should not be damaged - Be careful though! Watch what you're doing -
- Pull the brake booster towards you from the engine bay/ away from the firewall. You can use a helper here - get them to push the pushrod of the booster from the cabin to give enough clearance to get free. It may seem like there is not enough space but be persistent it will get out of there

Clean up the area whilst the booster is out and retain all gaskets - there should be one between the booster and master cylinder and one between the booster and firewall. If there aren't gaskets just cut some out with gasket paper, it's good to have them there. Especially the one between the booster and master, it isn't to seal it but prevents dirt and debris from getting in and wrecking the seal pushrod. This may be a cause of failure if a brake master was replaced/serviced and the gasket was lost.

Place your new booster in and reverse procedure, do everything up to spec. You can probably (at your own risk) reuse the nylock nuts that retain the booster if there is still bite when you screw them on (don't screw on by hand)

A note for Turbo owners
- On Turbo models a failed booster may be due to a faulty blocked or damaged one way valve. This one way valve sits in the booster body and connects the vacuum line to the booster. You can gently pry it out - chances are you'll need it as your remanufactured booster may not have one. To test simply suck on it (watch for oil :P), then try blowing through it. You should not be able to pass any air through it, if you can it's faulty and you need to source another one. (I don't believe N/A's use a 1 way valve on their boosters but it's worth checking. If you can't find a Saab one leave your damaged one in place and buy a generic one and put it somewhere in your vacuum line.

Once you've done all this go for a drive and prepare to be stunned by your pedal response, soft pedal and very sensitive brakes. It will feel like a modern car! It took me about 3 weeks to get used to the brakes. :D Enjoy

The other thing of note is that certain cars' brake boosters can be re-sealed at a common leak point, between the servo and the master cylinder. There is a seal, part# 8929010, which is available only from dealers now. It keeps air from getting into the booster from the joint between the master cylinder and the servo, and often is the only thing wrong with a brake booster that won't hold vacuum. Should be replaceable on all cars with the older-style master cylinder.

86-89 AFAIK (EPC) model boosters have that special seal, the later style brake booster came out with the 16v motor when Saab switched booster manufacturers.

You can use a bit of 2-3mm rubber sheet and cut one out (the stuff used as rubber floor/trunk liner) I just used a paper gasket, that seal is secondary, technically there should be another pushrod seal on the brake booster that prevents vacuum loss.

Quote:

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Originally Posted by s900t8v

86-89 AFAIK (EPC) model boosters have that special seal, the later style brake booster came out with the 16v motor when Saab switched booster manufacturers.

You can use a bit of 2-3mm rubber sheet and cut one out (the stuff used as rubber floor/trunk liner) I just used a paper gasket, that seal is secondary, technically there should be another pushrod seal on the brake booster that prevents vacuum loss.

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Right, I don't even think my car has a paper gasket there. I always thought it was omitted by SAAB to avoid that nasty situation where brake fluid gets sucked past the master cylinder seals and into the booster through the pushrod seal. Without that paper gasket, it bad master cylinder seals would just allow it to drain, rather than filling up the booster. Maybe mine was there originally and has disintegrated.

But the part number I posted above is for the pushrod seal, which as you pointed out is the more important one.