Cannot get to Single user mode

I am having trouble with an Xserve (10.4.11) and need to 'fsck' the boot drive. Unfortunately, for some reason I cannot get the machine to boot in single user mode. I have tried a couple of keyboards but the Apple-S is just not working. As an alternative I was going to do an 'init -s' but Apple has apparently hidden that command somewhere. Therefore my question is, how do I get this silly machine into single user mode? TIA.