[Richard]

Here you go. I think you should already have the three way valve. The concept is pretty straightforward. The valve normally allows air to pass from supply to the seal. When voltage is applied the valve shuts off the air supply and opens the seal to atmosphere. 

When using a vac pump, the vac is applied to the outlet that would normally be open to atmosphere. And the signal to valve is siamesed to run the vac pump.