the switch on the manifold brings ground to the valve.
Just disconnect the socket of the manifold switch and bridge to ground. Then you should hear the click of the firewall valve.
Or: disconnect the socket of the firewall valve and test the pins. The BL/YL/RD should give you +12V the BR/BK, leading to the manifold switch should give you nothing until the shut off pressure is reached.

To believe that the mainfold switch is o.k. is not enough in my opinion.
I feel better being sure it works.

Tom