Just a quick question - is there a knack to replacing them? I've got the old ones off but the new ones don't seem to want to just push on & I'm reluctant to use any mechanical assistance as they only appear to be made of a plastic/nylon type material.

Always apply with the valve already in place, lubricate with engine oil, I have used for many years a 'favourite' 10mm (12 point, 1/2" drive) socket to push the seal into place, I have, of course, damaged seals whilst learning the knack!

Thanks Roy, quick & professional advice - you cant get better than that!