2007 Silverado Possible Vacuum Leak?

[bradfordp2612]

Just bought a 2007 NNBS Silverado 1500 and when I bought it the dealer said that the master cylinder and the air intake system had just been replaced. I got in the truck and noticed that the "Service Brakes Soon" light on the dash was on and I asked them about it and they said that the EBCM was bad and that they would eat the cost of a new EBCM on the selling price, so I bought the truck.

A couple of days later I noticed a hissing sound coming from the brake booster/master cylinder area of the truck. I also notice that the truck is idling rough but I don't have an CEL and the hissing is very slight. What could the possible culprit be? I assume its possibly the master cylinder leaking into the booster, but before I go taking it apart, I wanted to know if there was a more likely solution to the problem.

The brakes also have a slight squeal when coming to a slow stop, but only momentarily.