Hi Gerrit,
Assuming it's running and pulling normal power, perhaps the
brake servo is leaking air/vacuum?
I say this because applying the brakes is actioning the servo, before that
there is no fault apparent.
There could well be a leak elsewhere as John has suggested, but that type
of leak would not change when the brakes are used.
I suggest you try blanking off the pipe to the servo, and driving it.
Obviously extreme caution needed as you'll have a very heavy brake pedal,
but if it doesn't stall you'll have an answer.
Go carefully
Nigel
