c-60 brakes locking up

hello

i have been working on a c-60
chevy truck flat bed
this actually my work's truck
anyway the brakes lock up
it hase drum all wheels
and it was locking up passenger rear
i adjusted wheel cylinders
the rear have two wheel cylinders upper and a lower
never worked on these before
is there a special way to adjust these
i've move star wheel in and out to try to get them adjusted and to keep them from locking up
but nothing works
just wondering if any body knows more about this than i

was wondering if it could be the porportioning valve
or master cylinder
never have seen this problem before
first time working with duel rear wheel cylinders and full drum brakes

any help would be a plus

thanks

you know what... I had the same problem with my 73 c-10 pickup before i took it apart to restore it. I had just taken the bed off and took advantage of the no-bed situation and tried some powerbraking with no weight on the back. it was pretty cool and all with the posi, but soon after i noticed that whenever I would press the brakes more than just a little bit, the right rear wheel will lock up and just drag on the ground. I thought it might be a proportioning valve or something but never found out for sure because its nothing but a black painted frame right now. once i get an engine and some brakes on it, ill start troubleshooting it. probably broke something like maybe the mechanism that adjusts the pads when you apply the brakes while rolling backwards. i do find it odd tho that both our problems were the same wheel(s)

you know what... I had the same problem with my 73 c-10 pickup before i took it apart to restore it. I had just taken the bed off and took advantage of the no-bed situation and tried some powerbraking with no weight on the back. it was pretty cool and all with the posi, but soon after i noticed that whenever I would press the brakes more than just a little bit, the right rear wheel will lock up and just drag on the ground. I thought it might be a proportioning valve or something but never found out for sure because its nothing but a black painted frame right now. once i get an engine and some brakes on it, ill start troubleshooting it. probably broke something like maybe the mechanism that adjusts the pads when you apply the brakes while rolling backwards. i do find it odd tho that both our problems were the same wheel(s)

i took apart the rears
all springs look good
pads are about 1/2 inch thick(good)
seals look good
i think booth rears are locking now
i adjusted the self adjusters out manually
both rears and the fronts
maybe the fronts arn't working correctly?

but when i first started
the problem was just right rear