Expired cat seat... but what if it's also a belt-positioning booster?

I know car seats expire, plastic gets brittle, etc and I'm fully on board with safety compliance. But I have expired car seats that also function as belt-positioning boosters. I'm curious... would the expiration have any bearing on their function as boosters if their purpose then is solely to get the kid's arse high enough off the seat to use the real belt safely?Sorry if it's a stupid question; my oldest is still in the car seat stage and I have no practical experience with boosters yet. Just wondering if these are worth holding on to for next year or using one as a backup (as booster only, when the time comes) in the second car... via /r/Parenting http://ift.tt/2aeNYPD