Brake servo Vacuum hold time.

[zippy500]

Random question.

I just setup the electric vacuum pump to the cars original brake servos to check if vacuum can be made. the car is 51yrs old now ( 1973 bmw)

I was quite happy as the pump did shut down , it lasted about 2mins before restarting again, ( not using a reserve tank, straight to the servo) thought that is was good, but not sure now ?

In a ICE engine the vacuum is constantly being made when the engine is running.

Is it good ?

If you have this setup in your build , does it do the same as mine

[jrbe]

Brake boosters are supposed to hold enough vacuum at least a few presses of the brake pedal to help you push the brake.
If it kicks on every 2 minutes that means you have a leak and your pump likely won't last very long. Our your vacuum switch doesn't have much hysteresis.

A reserve tank will lessen how often the pump has to kick on but it will also cause it to run longer.

My advice is to figure out the leak first then figure out if you need a vacuum reservoir. Could be the booster, check valve, hoses, etc.

[johu]

On Touran the vacuum is still there the next day and the pump only comes on when touching the pedal.
On Audi the pump comes on every 30s and is now causing issues. Either the motor current draw is increased or the switching FET is overheating

[FFMan]

for whats its worth, i started with a small reservoir, but deleted it and run the pump direct to the servo. It kicks in as you would imagine, after most presses of the pedal and on start up.

Oddly when the weather is cold it kicks in more often. I suspect a slight leak somewhere.

[arber333]

Well there are some limits up to where it pays off to have a really good performance and good efficiency from devices. Vacuum servo just isnt that in my opinion! It was intended to be operated in harsh enviroment and still perform despite potential leaks... t is just not the airlock technology in space. If the car is old and vacuum is leaking you can buy new bellows for it and try if it works.
Immediately after you run the pump (and switch stops it) you should have enough vacuum to activate brakes three times.
Otherwise it is just not worth it to complicate the beautifully simple machine that is EV.