Is it possible that the brake servo is at fault? Given that every other hose on the manifold is in good order. Stupid question but have you checked the breather hose going from the filler to the intake? Also the dipstick filler tubes both at the top and at the sump?
Edit: Could it be a valve not closing properly? Can you try an leakdown test on all cylinders?
Edit: Could it be a valve not closing properly? Can you try an leakdown test on all cylinders?