expert opinion needed please

[FOX5D]

Just got my s3 back after rolling road tuneup and noticed a strange (but probably obvious) problem. 2 things I noticed, namely: my servo assisted brakes didn't seem all that servo assisted; when standing at idle (about 900), when I put my foot on the brake pedal the revs dropped by 3/400! Obviously the vacuum has something to do with this but I am not educated enough to understand, and my tuneup man is away for a week. Can anyone hazard a guess or explain please?

my s3 has a sprint spec motor with webers and all the necessary reconditioning has been done.

Thanks

Jim

[pereirac]

Hi Jim,

Are you loosing brake fluid? The servo diaphragm might have split? Do the headlights still pop up smoothly, if not there may be a problem with the vacuum line?

Carl

[elans3]

I would check the non return valve between the inlet manifold part of the head and the lighting vacuum units. I had a leaky one once and that took me an age to find.
Steve
elans3

[FOX5D]

thanks Steve and Carl

the lights come off a separate non return valve on the manifold and they are fine. I've checked the non return valve in the servo and that is ok, although the mounting of the valve in the tank is a bit fragile-it is a lockheed servo and the valve is a push fit into the tank with a sponge sort of gasket. I think that this must be the problem as I have checked all the jubilee clips and the hose integrity. The brake fluid is also fine.

thanks for your help.

Jim

[wojeepster]

Your vacuum booster is bad.

[FOX5D]

Hi

I wasn't aware that I had a vacuum booster. There is a straight through piece of pipe that screws into the manifold to which the servo rubber pipe is attached, then there is a non return valve that goes into the tank on the servo.

The headlights vacuum operation is excellent coming off a separate non return valve.

Best wishes

Jim

[Emma-Knight]

dropping revs at braking indicate the problem is caused by the brake vacuum system - air from the brake system is fouling the mixture to lean. (vac picup is behind carburettor)
The no power assist brake does also show a point here. As the vac line towards the brake is always kept more or less under vacuum, a leak here would always affect the fuel mixture. So the problem should relate to the brake only. Maybe the short line from the brakes vac valve to the servo drum has a leak or slipped off-If the brake servo has a large clamp around the membrane drum, loosening and retightening sometimes might cause an air leak as well.

Is what Anna thinks at 3 o'clock in the morning

[pereirac]

My Sprint only has one non return valve in the vacuum line (on the inlet manifold) so when I had an air leak in my servo it affected my headlight mechanism. There was a non return valve in the servo but this never worked very well either.

Carl

[Frank Howard]

I wasn't aware that I had a vacuum booster.

Jim,

Vacuum booster is an American term for servo.

[FOX5D]

thank you everyone. i feel sure now that my problem is the non return valve in the brake servo so I will get that sorted and get my S3 moving (and stopping) again.

Thanks

Jim