This is done like the suction method, but, of course, blowing. Sometimes this will reveal a leak and a friend may be needed to find out the location. Or sometimes it may show that a spring is too weak. There is very little pressure when the clarinet is played, so weak springs are usually not the problem. See the smoke method below.
Check pads with a leak light:
I have found that many store sell a LED flashlight where the bulb is on a long flexible tube/cable. Harbor Freight has a model that has a nice long flexible tube. A leak light is handy for checking how the bottom four pads seat and the cooperation of the low F and E keys. But many smaller leaks are impossible to see with a leak light. I was taught by a man who often blew his cigarette smoke through a closed clarinet. Sometimes this can show the location of bigger leaks. But I don't smoke, so that isn't an option.
Check pads with plastic sheeting:
You could try sealing some holes by inserting something like thin bubble wrap or perhaps thin foam sheeting used for packing material. It needs to be something soft that will easily seal the hole. Have a friend hold the key down or rubber-band the key down to compress the plastic and seal the hole. If you find that there is a sudden improvement when testing suction, you have identified your problem location.
Checking pads with a feeler paper:
This is the best method for finding small leaks, but it takes some time and practice.
Get a stick, like a chop stick or tinker toy stick. This is your handle. Buy a package of cigarette paper (near the check-out counter of most grocery stores), non-gummed if possible. If you buy the gummed kind, cut off the gummed edge completely. Cut a long triangular fragment of cig. paper, and attach the non-pointy end to your stick somehow, like with a small rubber band. Stick the point of the cig. paper under a pad, and gently close the pad. As you pull the point out, you should feel drag or resistance. Do that “feeling” all around the pad: Put the point under the pad, close the pad, and pull the paper. Do this from at least 4 positions for every pad. When you find a place where there is no resistance when pulling it out, you have found your leak!
It's really pretty easy. Try it on a few different clarinets, and you'll get the hang of it.
Finding leaks with smoke:
Since I don't smoke, I never use this method. But the music store owner who first taught me a few things about repadding clarinets showed me how useful this is from time to time. Blow smoke in with all the pads and holes sealed. The smoke will come out of the leak. It may be possible to use soapy water similarly, and a bubble would come out of the leak. Probably this would be messy but effective. I haven't tried this yet.
If you find a leak, you may be able to help it by rubber banding the pad shut for 12 hours. If the pad works in combination with others, doing this may require further adjustment of the other key linkages involved.
If the pads all seem to be sealing, but there is still a leak, check these things:
Other things that cause leaks: