i would like to know:

- if you press quickly and fully the brake pedal 5-6 times while the engine is at idle, will the engine stall ?

- if you disconnect the brake vacuum line from brake servo unit while the engine is at idle, will the engine stop ?

No and no to btw of those questions

In theory it makes no difference if the brake servo vacuum line I connected or not, there Is a non return valve in the hose which only allows flow towards the engine..

If your engine is stopping then you probably have the non return valve fitting the wrong way round.

yes it is possible for the engine to stall as pumping the brake pedal is allowing extra air into the intake manifold, ONLY if you have a lean idle setting tho (pumping the brake pedal usually it will bring the rpm up slightly when the mixture is ok)

same for taking the pipe off while the engine is running you are allowing extra air into the system which is making the fuel mixture weak.

so, the one way valve lets air flowing towards the engine.

that is equal to higher pressure from brake booster tries to equalize lower pressure in the intake manifold.

in other words, for each pedal pump, a manifold vacuum leak occurs.

and since my car has a carburettor and no ECU, there is nothing to compensate for that...

yes basically that is it.

I think I solved my problem. but I need you to look it over.

what helped me most was a cross section photo of a vacuum brake booster (VBB) found on 2011 Encyclopedia Britannica (see below) and a great movie send by a friend that I uploaded on youtube at http://www.youtube.com/watch?v=KEKMsTAZss4

here's the thing: with engine running, vacuum is produced on BOTH chambers of VBB.

when the brake pedal is depressed, a poppet valve allows atmospheric pressure to fill the REAR chamber of VBB, thus creating a supplementary force for braking.

when pedal is released, the poppet valve is closed and vacuum is restored in both chambers.

the ONLY role of 1-way valve on brake vacuum line is to keep vacuum in VBB after the engine is stopped, so you can have at least one assisted braking.

my conclusion about my initial 2 questions:

- quick pedal pumping at idle stalls the engine because atmospheric pressure rushes into the front chamber of VBB due to a leak (diaphragm punctured or bad seals) so air/fuel mixture is leaned beyond limit

- when brake vacuum hose is pulled out from VBB, atmospheric pressure finds the 1-way valve open (this is normal) and totally leans the mixture -> engine stops.

now I will go make skoda dealer happy buying a brake booster.

Edited 16 November, 201114 yr by masster