66 Brake Issues

[C1066]

I have a 1966 Chevy C10 short straight bed with a 4.6 4 speed on the floor.

Purchased
front disc brake kit with drop spindles (installed)
new power booster with new dual master cylinder and prop valve with new brake lines (brotherstrucks)

I removed the old master cylinder with manual drum and drum brakes, and installed the power brake booster. I bench bled the dual master cylinder with the kit provided, attached the new lines to the prop valve to the front and new lines to the rear all the way to the T. Attached the brake pedal rod with no problem. Connected the vacuum line from booster to intake behind carburetor. Filled reservoirs with brake fluid and bled brakes to all air is gone, started from right rear to left front..felt normal when bleeding while the truck is off. (did this several times to make sure) Turn on the truck, press the brake pedal, but the pedal doesn't return unless I place my foot underneath and bring it back to normal. Even if I just tap the brake, it will go down however much I press, but not return at all nor keep sinking. Just stays there exactly when I let my foot off.

When the pedal is down, the truck will not move. When the pedal is returned to normal (with my foot), truck moves freely. I'm not sure why the pedal is not responding or what can be the root cause. I've bled the system several times, and the only thing I notice strange is when I do press the brake pedal, you can hear a swoosh, as it sounds like air is escaping near the pedal linkage.

Any help is appreciated. Thanks.