brake pedal needs pumping

hey guys

car is a 71 duster, originally a /6 car with manual drums
last week in installed a 8 3/4 rear end with 10 inch drums
I installed new cylinder and hard lines in the rear and let everything else alone
we topped off the brake fluid and bled the lines until a steady squirt came out each time I opened the bleeder (first the passenger rear, then the driver side)

the pedal felt good after I bled it

when I started driving the car I noticed the first time I hit the brakes the pedal goes to the floor, then I pump it once or twice more and it becomes hard

it seems like when the pedal gets hard it specifically want to lock up the passenger side rear

did I just do a poor job bleeding it or is there something else I should look at?