This'll be an easy one for you experienced Disco gurus...

Disco 1 '95 - I just replaced the headlight washer valve (T piece) under the bumper. When I tried to test them no water at the jets, and it looks like the headlight washer pump does not run.

Before I get too excited pulling stuff apart, is there a secret to getting those washers to work? I picked up that the headlights need to be on for it to work - is this correct? (I tried that but it didn't help).

Yes headlamps on, and will work simulteneously with the windscreen washers although there is a timed splash of water for headlamp washers, try a few times to get the water through the pipe.