CD I'm saying that the booster is leaking vacuum air. Inside the big round canister is a rubber diaphragm, engine vacuum is maintained on the front side and when you push the pedal rod a mechanism applies the force of the vacuum pulling on the diaphragm to increase the pressure against the master cylinder hydraulic piston, hence "power brakes". The linkage or push-rod goes through the center of the diaphragm with a seal to keep the vacuum from escaping. This seal has failed, it may be leaking all the time but when you push on the brake pedal the seal is shifted to a point where it really leaks. This is when you hear the noise of air pushing through this defective seal. They can whistle, howl, whine, roar, etc. at random. Having a stiffer than normal pedal in another symptom as the amount of power assisting vacuum is constantly being bled off through the leak. The vacuum is plumbed from the intake manifold itself, it's the large rubber hose just above and behind the throttle body. The check valve is located in the plastic fitting on the booster the hose is on, it holds vacuum when the pressure in the intake is high. There is normally enough vacuum stored in the booster for 3 stops without being replenished by intake vacuum. Air is removed from the booster and it stays that way. Vacuum itself is used for assist and air is not, so there is no significant air flow into the intake without a leak. The fuel injection will set a lean code when in closed loop the leak is bad enough for enough time, like hot engine at a long light or at a drive through window with the car in gear.
As for the fluid leak, see if you can dry it with brake clean then go for a drive or hold the pedal hard for a while. Watched points never boil and drives seem to work better than hold and wait on slow leaks If it's leaking you'll have to attempt to replace the bleeder. New ones are available at the parts store. If you can't get it out without destroying the threads in the caliper just replace the whole caliper. After the repair bleed the air out at that wheel.