black & white, problems in photoshop 5

To my horror I discovered this problem while making black and white illustrations with photoshop: I can't make the background pure white! Even if I put the cmyk value as 0000 it changes it automatically to 0223 or something. You can't tell the difference on the screen, but in print this comes out as a very light grey, which pretty much destroys my work. I don't think the black is pure black either.
Working in bitmap mode is not an option.

Can anyone help, please?

are you editing a file saved in a format such as gif or jpg? sometimes when editing saved image files, they were exported without some colors used and when you go back to edit them it wont let you use colors that were not included in the export, but instead only what was in the export pallet.

if this is the case, try copying your files to a new project. should fix the problem