If your trailer (any modern trailer) has dual chamber spring brakes, you might have a bad brake pod. When you supply air to release the brakes (compressing the brake pod spring), the air you hear may be escaping from the pressurized 'release' chamber to the non-pressurized service brake chamber via a bad rod seal. This non-pressurized chamber is vented through the brake valve so that is why you would hear air escaping from this valve.
When the system is operating normally, there is air holding the spring in the compressed position. When you step on the brake, air fills the service brake chamber, which pushes the rod hooked to the slack adjuster and applies the brakes. When you let off the brake pedal, the brake valve dumps the air pressurized in the service brakes, which allows the rod to retract and release the brakes.
To find the bad brake pod (if this is the case), pinch off the brake release hoses going to each brake pod. Supply air to the trailer and make sure there is no air escaping from the brake valve vent port. Unpinch the brake hoses one at a time, when the air starts escaping from the brake valve, that is the bad brake pod.
Hope this helps!
