Simon,

They shouldn't be brittle. They should be made of UV resistant polypropylene or similar with a small amount of talc. If they have lasted this long and not cracked or snapped they should be good for some time yet. Try taking the complete assembly out then check that the seal works by visual inspection. If OK then simply re-silicone the complete assembly to the deck then wait for it to set.

If it ain't broke then don't replace it. I don't know about the quality of some of the newer plastics but from what I have seen come in from overseas the UV resistance isn't what it's supposed to be. <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

Regards,

Peter