Here is a technique I have used for many years on cracked or pin hole leaks in a bellows. It is VERY effective.
Go buy a can of the liquid Plasit-dip in black. Read the instructions and buy the thinner/cleaner (can't remember offhand which solvent it is). Get yourself a good quality 1" paint brush. Now, thin the Plasti-dip down to a thin consistency. Remove the bellows if you can. If you can't be sure eveything but the bellows is well covered. Make sure the bellows is free of dust or any foreign matter. Very lightly paint the problem areas with a THIN layer of the Plasti-dip. Let dry for a few minutes and then work the bellows when it is at a point that the Plasti-dip will not stick to itself. If you have thinned it properly this will seal all your leaks and yet leave the bellows flexible for use. I have used this technique on everything from MP-4 cameras to Old Sinar P's and it has served me well. The only down side is that you must do it on the outside of the bellows because the end result is fairly shiney and so it shows. Inside the bellows it might cause glare.
Hope this helps.