Brake servo or vac leak

[EDDY]

Mk2 syncro,weber carb.
Something is wrong . If i pump the brake pedal the idle speed increases and the engine seems smoother. I guess i have a vac leak at the brake system.?
Is there a simple way to test the servo ? I dont think its the dizzy or the syncro vac. The 4wd does work in reverse []

Also, if the pipe for the brake servo is blanked off , will the brakes be even worse than standard ?

Thanks chaps.
Ps. DMTR 34 carb, had a new DMTL 32/34 and was not as good. Not much difference but def found the DMTR more tuneable Etc.

[ld50]

could maybe be one of the little one-way valves?

[Chris S]

If you put your foot on the brake pedal & then start the engine, you should feel the pedal drop a little, if not then you might have a vacuum leak in the servo,

if you blank the servo off then the brakes will work just the same but you will have to push on the pedal a lot harder,
think of the brake servo doing the same job for the brakes as power steering does to reduce the effort required to turn the steering wheel.

[EDDY]

I blanked off the tube at the side of the in-manifold and it made no difference to the running at idle, and after sucking on all the tubes attached (brake serv, 4wd res, dizz advance) i could’nt feel any leaks ? One way valves seem ok.
Tried the brake test and the pedal moved down when started.

Would be interesting to know if any one with this set-up (weber 34dmtr or standard carb) finds the same ?
It only changes idle speed if you continuosly pump the brakes. I can live with it tho,

Thanks for the replys.

[alexisblades]

It’s going to change idle speed if you pump the brakes continuously. What happens is, the engine sucks air from both sides of the brake servo diaphragm. When you press the pedal, it allows air into one side, thus leaving vacuum on the other side. The resulting pressure difference is what provides the braking assistance. Continuously pressing the brakes means you’re effectively creating a continuous vacuum leak into the engine, which would under normal occasional use of the brakes be only an occasional leak.

Note that an engine with an ecu which uses a manifold pressure sensor to determine engine load should be able to add fuel to match the extra air which is entering the engine, so you won’t run lean. With a carb this isn’t the case, you will run very slightly lean just after you operate the brakes each time. If the car runs better when you’re continuously pumping the brakes, it could indicate that you should get your idle mixture/speed checked and/or adjusted.

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