flash linked through X link socket only fires on AV and P mode

HI, I am trying to get my vivitar 3600 flashgun to fire with the K20 connected through the X synch but it only fires if I choose Av or P mode.
I am trying to use the M mode to choose the aperture and the SS but the flash doesn't fire unless I use also the onboard flash.
I would like to fire it without having my onboard flash, am I missing something here please?
Thanks for your help

Amin

Set the camera to X (flash sync).

The speed will be fixed at the sync speed, the flash will fire every time and all you need to adjust is the aperture.

thanks hefty, will give it a go

Have you tried M mode with shutter speeds less than 1/180s?
If you go above 1/180 the PC flash shouldn't fire (because if it did you'd get nasty black bars on the frame!)
Matt