You mean you hear air in the cab down by the pedal. If thats the case your power steering assist is leaking. Its the big black unit behind the brake master cylinder. It may set a check engine light at times for lean condition.
There could be air in your lines if you know how cheak your brake pads to see if there are low( if so replace the pads) fill brake fulid and bleed the brakes this should fix your problem
SOURCE: Hissing noise when releasing brake pedal
Sounds like your brake booster but can't be sure until I know what kind of truck or car you have....
SOURCE: on occasion front brakes staying engaged while
Well, guess I don't need to tell you this is dangerous and that your front Rotors are probably trashed.
This sounds like either a bad Master cylinder or a leaking Brake booster. There can also be a bad check valve on the Vacuum line plugging into the Brake booster. There is a rather large line with a plastic fitting, sometimes a 90 degree fitting on the end and the check valve is in the plastic
You can test the Brake booster with the engine running and using an approved Brake cleaner safe for rubber and plastics. Cover the outside bottom of the hood so spray does not get on your paint. Then spray around the Booster. If the engine RPM changes, you have a leak. Save the actuating rod, and replace the Booster. Spray the check valve, if RPM changes replace it.
Now for the Master cylinder, pressure gauges can be attached to the front fluid discharge to measure if the internal pistons are sticking. But you may spend more Labor in testing, than the part is worth.
My reasons for choosing these parts as defects are that your "Brake or ABS" warning lights are not on so the problem would be before those monitoring devices. The Vacuum system is not monitored and you hear hissing. The Master cylinder is not monitored before the ABS unit.
I hope you find my solution helpful.
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