The most popular light leaks i repair are the film back n rev back seals. Its very rare the dark slide baffle causes leaks because its a spring steel flap with some felt but will seal even if the felt is completely worn.
The other light leaks to look for are at the corner folds of the bellows.
Buy the foam seal kits fron jon goodman on ebay n diy. Its a very simple n inexpensive fix.