I recently got a 9' 5wt fly rod kit from Janns and didn't like the graphite reel seat at all. So i picked up a cork sliding band seat. As i look through all the beautiful rods produced, i've noticed that the cork seats are usually only on the really lightweight rods. So my question is simple. Is a cork reel seat functional on a 5wt rod or would it not be strong enough? I would hate to put it on and have it break under the stress. Thanks

The cork sliding band seats are usually selected for their very light weight, hence their appearance on lightweight rods. There's no reason you couldn't use one on a heavier rod, as long as the sliding band(s) secure the feet on your particular reel properly. Even some lightweight reels have feet that are taller than typical, and work loose when used with sliding band seats. I'd suggest you check to see how the reel you plan to use fits prior to mounting the seat.
Gnossos

It is not uncommon for older bamboo rods to have cork seats with either a pair of rings or a cap and ring. Most of these were 6wt or heavier and 8 foot or longer. I have an Orvis bamboo with a cork seat in a thread lock which is an 8 foot that Orvis marked as 7 wt but everyone agrees is a 6.
I think you will be fine. The caution about fit is a good one. I would add that the rings should be sufficiently wide to distribute the forces over as much cork surface as is reasonable. I have a Lee Wulff Midge which is a 6 foot bamboo for a 7 wt line. To get things under the magical weight of 2 oz. they used rings which are only 1/8 inch wide. A reel big enough for a 7 wt line, forcing 1/8 inch rings into the cork seat Not the best and as you can imagine the cork is pretty bad. Another project.
AgMD