Ford brake booster problems

Hey, im finishing up a 68 galaxie and ive got a problem with the brake booster. Its an original unit thats been rebuilt. When the engine is running and I step on the brakes, the booster makes a squeaking/sucking sound. The sound comes from the back of the booster near the firewall. Where the rod goes into the booster, there is a rubber boot and when the brake pedal is depressed it sucks the rubber boot in and that is what makes the squeaking sound. Hopefully its an easy solution that my brain just isnt registering.

Thanks,
Chris

 

Front and rear are two chambers separated by the rubber diaphram. If you are getting air sucking in continuous on the back side, its got a leak. If it was a recent rebuild under warranty, swap it out.