It seems to me that you are barking up the wrong tree.
If you have the stock single outlet master cylinder its
only outlet goes to the booster and then on to the splitter
where the famous hydraulic brake light resides. If the
master cylinder was hanging up then the front brakes would
be locked up too.
I think the problem lies in the rear brakes. In fact I would
look strongly at the brake RETURN springs. Remember the
wheel cylinder presses the shoes out, and the spring pulls
them back. The springs may be weak or broken from age.
Perhaps someone put the springs on the wrong side of the
shoes or used wrong replacement springs. The brake springs
could be hanging up on something.
Try this test: Wrestle a drum off. Then put a pipe clamp
cross the shoes and leave about 1/4 inch between the clamp
and the shoes. Have you buddy press on the brake and
watch the shoes move. They should be held firmly by the
clamp. Then release the brake and the RETURN springs should
pull the shoes back.
If everything looks good, then is another REMOTE possiblity.
How old is the single rubber brake hose. It may be broken
down inside, allowing fluid to go out but not return freely.
A remote possibility.
Personally I would the take brakes apart, clean everything,
lube the backing plate at the proper points, and then
reinstall correctly. I would do this before I would look
at the master cylinder.
IMHO
Dave
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