A cuckoo clock will happily run and strike without bellows but it is the bellows that create the unique sound.
When a cuckoo clock requires servicing the bellows should be inspected and if they are in poor condition they should be replaced. The constant, every hour and half-hour strike of the clock combined with the actuation of the bellows causes leaks at the creases as these are the highest wear points.
So, how does one repair the bellows?
Carefully take the old bellows tubes out with a hobby knife. On some cuckoo clocks, you may have screws that hold the bellows in place. Once they are out resist the temptation to rip them off the wood columns as the bellows themselves can be used as a template for your new bellows.
Here are the basic steps
Make a tracing of the bellows opening on paper. Scape away all the old covering and old glue with an Exacto knife or similar. Using TYVEK, cut a piece of material approximately 2 inches by 7 inches. On the fixed end of the two bellows pieces glue a new hinge with a strip of TYVEK.
Using the template you made earlier and with the bellows open wrap the TYVEK around the bellows. After you are satisfied that it will fit, apply Seal-All or a carpenter’s yellow glue to all outside edges. The sides will have a V-shape. Press the TYVEK to the sides and ends of the bellows for about 2-3 minutes until the glue sets. Trim any excess TYVEK.
Wait for about two hours until the glue is dry. Using a popsicle stick or similar, start by gently making the creases pushing inward as you go. Press the bellows together and clamp (or rubber band) for 24 hours. The TYVEK needs to be formed so that it knows which way to fold when the bellows are actuated.
When done reinstall the bellows back into the clock case, connect the actuators, and re-attach the back cover.
If you hear a “Cuck” but no “Koo” you’ve done something wrong. Once you’ve done a few of these you will get it right every time.
Enjoy the unique sound of your cuckoo clock.