I put my foot on the brake and I hear a hissing noise. The brake pedals get really hard and it is difficult to stop. In addition, when I step on the brake pedal, the the engine revs a little bit higher. i need some advise- Thank you.

Sounds like a leak in the brake booster or vacuum line to the booster.

Could also be some air in the booster, i hade the same a while ago. It went over by itself. Try to de-air your brake installation, and check for leaks anywhere. It's sounds lyke your booster, which i cann make up on your story.

Yep, it's your master vac. Scrap it and get another one.

Regards,

The higher idle when pressing the brake pedal is the telltale sign that it is your MasterVac (brake booster).