'97 outback brakes "hard" to press

whenever the air temp is less than about zero Fahrenheit (O F), the brake pedal on my car becomes REALLY hard to press, as if i no longer have power-assist brakes.

i have checked my brake fluid level and it is fine. the serpentine belt is almost new and looks/feels fine.

the car is a 1997 legacy outback with a 2.5l and automatic.

any idea why the power-assist part of the brake system would not function in cold weather? since i live in AK, the air temp is often less than 0 F so i would like to diagnose this problem and fix it myself ASAP.

thanks

You might check the booster vacuum hose, the valve might be hanging up. Take it off and try to blow/suck through it, it should flow easily to the engine but not to the booster. Make sure to replace it with an OEM one if it needs it, that valve is important, it's not just a regular hose.