Holley 1920 issues

Are you perhaps jumping the gun on purchasing brake parts? I tell you what; you haven't isolated the front from the rear yet have you? If you wanna buy parts, buy a rear brake hose cuz after I tell you this next thing, you are probably gonna need one.
Put a clamp, hard, on the rear flex line. Bingo! the rear brakes are defeated. Now, engine off, do you have a hard pedal, at or near the top? If yes, we can move on. If no we will have to figure out why.
EDIT
Be advised the clamping an old hard rear flexhose, can damage it inside, where you cannot see the damage. Then at some future point, the rear system can go down, usually by locking the rear brakes on. I don't normally use this method, except as a last resort. I offer it here, because the rear hose is old(and thus probably due for a change), and because it kinda is the last resort, and also because it will expedite the diagnostic time.
There is/are another way to block-off the rear system. It just takes a sometimes hard to find plug for the end of the rear hard line, or a plug at the M/C end. Which then takes time to un-install.
When I clamp the line, I use a clamp with large rounded anvils; not a visegrip plier, in an effort to minimize internal stress/damage. Remember, the outer sheath you see is just a protective skin for the internal hose. Damage may not show up for hundreds of miles later.