upgraded to power brake system-brakes are very hard to press-takes both feet pressing really hard-replaced the new booster already too-What next?

Darn HoTRoDLVR27 ... The power brake booster would have been my first thought. My next thought is the check valve or it could be as simple as dirt in the lines. Bleeding the lines could be a silver bullet here. Try that. If that doesn't help try the supplier and/or manufacturer of your power brake system. They should be able to help you chase down the problem. Good luck!

Other things to check are the master cylinder and brake pedal push rod. If you have a dual one, they can leak between the cylinders - not sending enough fluid toward the slave cylinders. If the push rod is too short, pushing down on the pedal does not push in far enough into the master cylinder to force brake fluid into the slave cylinders. The too short a push rod is a common mistake in upgrading vintage cars, especially the likes of mid 50s (TBirds, Fairlanes, Chevy 210s & BelAirs, etc.)

You might have the wrong master cylinder in my case I had the wrong master cylinder and the pushrod was too long. I purchased the correct master cylinder and used the short pushrod that came out of my original master cylinder. I had purchased a booster that came with the master cylinder even though the Box said my year and disc / drum the master cylinder was for a different year hope this information helps