Brakes could be weak because of adjustment, arcing of shoes, primary/secondary shoes wrong place/ wrong side of car,

You really should use a single resivor master cylinder

If you are using a dual master , do you have the combo valve that equalizes front/rear?

Drum brakes need 10# residual valves to keep shoes near drums,

have you pumped the brakes(used to be common driving practice

did you bench bleed the master cylinder??????

1/4 inch lines won't help you