Your hydraulic brakes use a vacuum canister much like a car to provide the power assist most likely. The low vacuum light and warning buzzer are telling you something ain't right
My guess is the reason you're hearing the electric brake motor working is because it IS working because there is no vacuum. If your brakes are working just fine atleast you know the backup system is working correctly
I would check for a vacuum leak somewhere if I were you. While most buses with hydraulic brakes use the power steering pump to provide the power boost, I have seen a few medium duty trucks that still used vacuum with big tanks underneath. If you have a low vacuum light, chances are that's why.