1997 Ford Ranger brake pedal hard, air noise, not returning

Ever since I got a new motorcycle the truck has been sitting around, to the point that it has become alive with moss (we get a lot of rain and flexible temps, the truck is usually moist inside). I fired her up one day to do something and noticed that the brakes were extremely hard to push, so I just parked her and did whatever. The next day I noticed the battery was dead because the pedal had stuck in a position which engaged the brake lights. Just to note I’ve done no mechanical work other than an oil change in the last year.

I’ve done a few tests and noticed the following:

• Brake stays down after being pressed. The brakes themselves are still engaged at this point and the lights are on. It’s not a strong braking, but enough to know that you are indeed braking.

• When I push in the clutch and it returns up, the brake pedal returns with it.

• The brake pedal itself is very hard to push, as though it engages a bit then hits a wall.

• When the brake pedal is pushed a loud air noise can be heard.

• Leaving the engine on for a while does not help this situation.

• With the engine off and my foot on the brake pedal, it will go down a slight bit when the engine is turned on.

I think that’s it. According to the internet it’s either a vacuum leak or my brake booster is borked. I’m guessing the latter since the hose going to the booster looks great and the air sound coming from around the pedal is new, I think. However what I think is irrelevant since I’m not actually a mechanic, so I’m asking the internet. God have mercy on my soul…

EDIT: Results of tests in comments:

• The engine does not do anything different when pressing or not.

• I clamped the airhose and found the following:

  • When I pressed, no air noise
  • When the parking brake was off, the pedal would fully return, it would not when the parking brake was on
  • The pedal would not depress very far, and was very hard

• When I removed the clamp

  • The pedal went all the way down like normal for a few pumps
  • Same behavior as previously found, air noise came back.

I will go with Bens analysis and get a new booster in a week unless I hear otherwise. If you want to make it official I’ll accept your answer as the correct one.

Edit: Ben was correct, it was a failing brake booster. I replaced the booster with a new one and it solved the issue. I’m not sure what internally was failing in the booster and no longer have it so I can’t check.