BayAreaKiteboarding.com

Hi,
I started kiting last year and bought a really cheap 2009 Slingshot turbo II. I was hoping to get decent at kiting before upgrading. I went out today in some weak wind, and the kite fell down shortly after i got in the water, and someone told me and my buddy it was underinflated. So we pumped it up some more, and launched, and then it fell and sort of folded over itself, instead of maintaining its C shape on the water.

Do old kites lose shape in the bladder? I pumped it up and it seemed to hold its firmness well, but the guy at the shop told me it was underinflated. Am i just not pumping it enough? Is it leaking? I was hoping to use this kite until I'm decent before upgrading. Sorry, i know it might be hard to answer this question without seeing the kite in person.

Has anyone ever considered using green slime (for repairing leaks in tires) in a kite, or would it be too heavy?

If you inflate the kite well and it goes soft, there's likely a leak in one of the bladders.This is usually due to a hole in the bladder material, or due to a leaky valve.
It's a lot like a bicycle inner tube.

First try to determine how fast and where the leak is.
Pump your kite, close the struts and let it sit, Put your ear up to the leading edge and listen all along, as well as to the struts to tell if you have an "audible" leak, and especially check each valve. A fast leak is generally audible. If it's not audible, check the kite every ten minutes or so to determine how fast it is leaking and in which area of the kite. (leading edge or strut ?)

To find a slow leak you generally need to pull the bladder out of the kite, and check it underwater (a pool is useful, but I have also used a bathtub).
Watch some repair videos first so you have an idea of all the steps.
such as this one
https://www.youtube.com/watch?v=eBWovk0Kz0E
Be careful not to damage the bladder when it's out of the kite. Watch out for rose bushes, cactii, etc. and cats that want to play in your vicinity !

To patch a small hole in bladder, "Tear-aid" works great.
To fix a de-laminating valve, I have had success gluing valves on using contact cement. I use hot water to remove the valve completely before gluing it back on.

What he just said.
Also, get a gauge and look at your manual to know how hard you should be pumping your kite.
You won't need the gauge after a season of pumping since you'll be used to what the pressure should be, but it's a great way to make sure at first and an easy way to know if you have a slow leak since you can check if the pressure holds by just looking at the gauge every 15min.
Bladders are a pain to repair but it's not impossible.
However, if there is indeed a leak, I would just give it to sonny at third avenue and save some time.