Replacing drum brakes with discs can be a simple or difficult task depending on the kit you use. But the one thing some people forget to do is to remove the residual pressure valve from the master cylinder. Over the weekend I helped my buddy work on his 1960s Nissan Patrol. The brakes on it were shot and he couldn't find parts for it so he swapped in some Landcruiser axles that had discs front and rear. He swapped the master cylinder to one from an old Datsun truck which had drums all around.
So every time he hit the brakes they would remain clamped to the rotors and wouldn't release when you let off the brake pedal. I told him he needed to remove the residual pressure valves over the phone but he had no idea what I was talking about. So I decided to head over and help him out. In about 20 minutes I had the residual pressure valves out (both front and rear) and the brakes were working perfect. Well, as perfect as really rusty rotors will work.
Anyway, the valves are typically under the brakeline seats in the master cylinder. I threaded a wood screw into the seat and pulled it out. The spring and rubber plug are the residual pressure valve (sometimes it's a spring and a ball bearing). I tossed the spring and plug and put it all back together. Done.