rpm increases when i hit the brakes

[DA78westy]

I hit the brakes when idiling and the rpm's go up , engine starts "clearing her throat" like the mixture is leaning out. no vaccum leaks in engine compartment, but sounds like there is one somewhere.Brake booster? 1978 2.0 ltr, dual webber 34ICT.
Also, when i disconnect and plug vac lines between carbs idle drops by about 200 rpm"s. replaced check valve also.
Any advice or ideas?
Dominic

[Amskeptic]

I hit the brakes when idling and the rpm's go up , engine starts "clearing her throat" like the mixture is leaning out. no vaccum leaks in engine compartment, but sounds like there is one somewhere.Brake booster? 1978 2.0 ltr, dual webber 34ICT.
Also, when i disconnect and plug vac lines between carbs idle drops by about 200 rpm"s. replaced check valve also.
Any advice or ideas?
Dominic

If the rpms go up and stay up when applying the brakes, then the booster is failing.

The fact that the engine rpms go up as opposed to down, means the mixture is a bit rich.

What vacuum line between the carbs causes this rpm drop? A vacuum leak of sufficient size will drop the rpms of any engine, this would not be a rich versus lean situation as mentioned above, but a large vacuum leak.
Colin

[DA78westy]

I unplugged the line that goes to the booster, and then blocked off the lines from the intake manifold. by doing this the rpm's dropped.
the rpm's go up and then drop when hitting the brakes rpms go back up and then down when releasing the pedal.