Mushy Brake Pedal After Sitting - Moisture in Brake Lines

Brake pedal felt mushy/spongy after the car sat in an unheated garage for approximately 3 years

Moisture had accumulated in the brake lines during the 3 years of storage in an unheated garage, causing a spongy pedal due to the lower boiling point of contaminated brake fluid and possible air introduction from moisture absorption

High confidence — Original poster confirmed this resolved the issue