Why is my brake pedal hard when I apply the brakes?

Hello,

Typically, hard brake pedal is due to an issue with the vacuum system for the brake booster.
Check the hose that attached to the booster.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Make sure it is secured and does not have any restrictions or crack. It will be a good idea to look over all the vacuum hoses and make sure they are all in good shape.
Also inspect the check valve that is located on the brake booster (at the end of the hose). If that valve is stuck closed, you will have hard pedal.
Another possibility is a bad brake booster.

Let me know what you find or if you have any questions.
Thank you.

Thank you Sir.

You are welcome.

Where is this check valve located on the brake booster?

Thank you Sir.

Depending on the type of booster the check valve can look different.
Can you upload a picture of the brake booster?

My truck is in the shop having the carburetor rebuilt.

No worries. Those pictures are the two different type of boosters for that truck.