I am assuming you have drum rears....
The master is 2 parts for safety reasons if a line breaks on the front or rear system, the other will continue to operate.
Now the front disks supply 90% of your stopping power ast they squeeze then the power transfer to the rears for the other 10% and to keep it straight. When the rear drum shoes are too tight they will lock up since the fronts are "grabbing" yet plus the disk brakes have to fill and travel a bit before they start to grab, therefore the rears were hitting hard.
Since you backed them off a bit they shouldnt lock up unless you really get on the pedal in an emergency, Take it for a drive, get em warmed up a bit and test it by making various stopping situations in your mind (eg: slow normal stop, a soft stop, a hard stop kind of like almost missed your turn, and a HARD stop kinda like a kid ran in front of you ) did your truck handle as it should and you feel comfortable? Try again some day when its raining just to make certain and that you are comfortable with the way it feels and handles. If you have any doubts go to a reputable shop, better safe than sorry.