Sam,
Here is how I determined that my NSS was the problem. The method is courtesy of Arthur Dalton who writes:

"Here is another prof positive test that will test the same point , but in a different way..
Get a jumper wire and hook one end to the bat + ...
Now, unplug the connector at booster and I want you to look at the 3 MALE connectors at the part of the connector that is screwed to the wall..[not the part you unplugged w/ female connectors]
Take your bat + wire and touch the CENTER MALE connector .. the starter should crank the engine ..if not , you have a problem at the starter or wire going from this connector to the starter.. if the starter turns the engine over [ cranks] w/test, the starter/sol are verified as GOOD. This means you are not getting power at the wall 3 prong connector and the problem is in the NSS circuit.."

Sam
The connection in Mr Dalton's directions is near the brake master cylinder but located on the 'wall' between the engine and the master cylinder and vacuum booster. Disconnect the plug and apply the battery's voltage to the cnter male pin; a really easy test (you may have to put the ignition and turn to "run", I don't remember).

regards,
Mark