You can check they are leak proof by wetting your hand and placing one end on your palm and the other on your mouth and sucking to create a vacuum.Don't do this too hard or to long or you will end up with,"Mick Jagger" lips<BFG>
Another problem that causes Air leaks when the boots are fitted(Which will cause carburation problems like misfires/high idling/lean burn/RPM stumbles) is that the Synthetic Rubber they are made of goes hard with age.The only part here we are concerned with is the Ends Grooves where the clamps go on.
The one in the picture the ends were so hard as to make them unusable as they would not seal.They may well have been made out of hard plastic and therefore not flexible to be able to be clamped-to-seal.They had ZERO flexibility/Softness.

I previously tried leaving the ends soaking in Brake Fluid.Sort of worked but took a week and wasn't that good.Tried Silicone spray.Again sort of worked but not good enough.
Success
Tonight and in One Night I've brought them back to life.
In the picture above I used the small,"Pencil Paint Brush" and painted some

paint stripper on the outside and inside of the grooves.Did one coat and left it for an hour and as always with this kid of stripper it dries out.Then did it again and repeated it on the hour 4 times.
Then cleaned it off with cold water.The Boots are now nice and soft and can be used again
Note and Warning:You can usually get this type of Stripper at DIY stores/Hardware stores and doesn't have to be Automotive.Use rubber gloves:Keep well Ventilated and NEVER get any on your skin or in your eyes as it is corrosive and can cause Rashes and or Blindness!!!!!!
Wash immediately with cold water if you get ANY on your person or clothes!!!
Note:I've speeded up the process by leaving the clamp end of a boot in a small wide glass with some paint stripper in it.After around half-an-hour the boots expanded to a point where they looked unusable but after cleaned with water and left to soak in cold water the boots returned to their normal state but were pliable.The boots have been on my Main CX for over a year now with no problem.
If you require new Clamps(Old ones can also cause air-leaks),
Then see,
http://globalcxglvtwins.hostingdelivered.com/viewtopic.php?f=17&t=76

