Help with Master/Slave configuration (550ex and 580ex)

So I've been using an old 550ex on the hotshoe to trigger my 580ex off camera. It works, but I would like the hotshoe mounted flash to not fire it's bulb while still triggering slave, is this possible? I tried checking the custom functions in the manuals, but no luck.

P104 of the Instructions describes how to toggle off and on the Master Flash, without affecting it's coded instructions transmissions. It's part of the standard Master mode settings.