What could be causing weak brakes on a 1977 Ford F-350 one ton truck I have changed master cylinder twice brake booster 4 times replaced calipers and shoes hoses help?

1. Air in the brake lines: Air in the brake lines can cause a spongy pedal and weak braking. Make sure to bleed the brake lines thoroughly after replacing any brake components.

2. Faulty proportioning valve: The proportioning valve is responsible for distributing brake fluid to the front and rear brakes. A faulty proportioning valve can cause the rear brakes to receive too little fluid, resulting in weak braking.

3. Clogged brake lines: Clogged brake lines can restrict the flow of brake fluid and cause weak braking. Inspect the brake lines for any kinks or blockages.

4. Worn brake pads or shoes: Worn brake pads or shoes can cause weak braking. Inspect the brake pads and shoes for wear and replace them if necessary.

5. Faulty brake booster: A faulty brake booster can reduce the amount of assist provided to the brake pedal, making it difficult to stop the truck. Inspect the brake booster for leaks or damage and replace it if necessary.