Hi Pam, this is typical of the Clavietta, it is an inherent issue that they all have. If you are getting 15-20 seconds of sound you are doing actually pretty darn good. If you believe you are getting leaks from the gasket, first try this. Mix some dish soap and water and with an eye dropper place a little around the air chamber and blow with out depressing any of the keys and see if you get any bubbles. If you do not and you want to see if any air is leaking from your reeds due the same. place the soapy water around the reed plate and not on the reed. Do a few reeds at a time, this way you can concentrate the soapy water in a specific area. Repeat until all have been tested. If you get bubbles in the air chamber try adding a little silicone on the gasket in that area,let dry 24-48 hours before you reinstall your chamber. If you are getting an air leak around the reed plate, mark with a grease pencil. Order some accordion wax, you can find on Ebay. Note: (Not regular bees wax). Cut a small piece on place on a spoon heat on stove (use mitten as the spoon will get hot. let it melt and carefully pour at designated area you have marked let dry and it should do the job.
Hope this Helps