2001 Tundra Mushy Brakes

Chronic Mushy Brake Pedal. I am Lost.

When I first got the truck, the brake pedal was both firm from the top and it applied the brakes all the way through the motion. Starting this cold season my truck's pedal became much easier to push and only applied the brakes in the last quarter of pedal travel. I am not sure what is causing this. I have replaced the master cylinder, bled the brakes, and checked for leaks. After the master cylinder and brakes were bled, the pedal felt right for about 30 seconds then back to the issue we had before. If you have any ideas, I would really appreciate it.

worn brake booster, but they cost a ton and often just stay a little mushy and never get worse

Thank you. I really appreciate the quick reply!