Driver side hissing when I hit brakes??!

[mustangman289hp]

So I just got done installing some lowering springs and when i first started it again and pushed the brake pedal I heard a hissing sound. It only happens when I push in the brake pedal... even just lightly nudge it. Could this have been caused by messing with the caliper during the spring install? (just zip tied them out of the way). What could this be? Thanks in advance I appreciate it.

 

[Vinnietbird]

Could be your brake booster going bad.

 

[Mustang5L5]

Check brake booster for leaks

 

[mustangman289hp]

Could it fail on me while driving? Its weird though cause I didnt notice it before