2003 Caravan low brake pedal

Only days after garage installed my winter tires, began to notice the brake pedal dropping lower and lower to the point where it was almost to the floor. Brake fluid level was fine. No strange noises. It has ABS brakes. No instrument panel lights were coming on. Brake shop replaced the master cylinder and bled the lines (they did find some air in the system). Seemed fine for first few days but then the gradual brake pedal dropping started all over again. Took it back to the shop. They bled the lines again and found more air. They want to start replacing more and more parts starting with the calipers (like a fishing expedition). Please help. Someone mentioned it could have something to do with ABS module that can let air in if not functioning properly??