Usually this is symptomatic of a bad residual check valve, which is located in the master cylinder. It is the last thing you installed in the master cylinder before you screwed the end on. It is the piece that looks like a helmet with holes in it, that the return spring rests on. Take the master cylinder apart again and inspect this carefully.

Also make double sure you have ALL the air out of the master cylinder and brake cylinders.

Yet another thing to check is that the brakes are adjusted properly, ie. very lightly touching the drums. You should be able to jack up the wheel and hear the brake shoes just barely touching the drums as you spin the wheel.

Hope this helps!

Remember 9-11-01--God Bless the USA
JUSTICE, not REVENGE, will prevail

1951 Chevy 1/2-ton Pickup truck