Lubrication on the pivots, brake cable and controls would solve this problem. Start with a light oil (30w) on the pivots that extent into the brake cover, then get a spray can of cable lube down the cable or, get a plastic bag, remove the cable, sheath and ferrule from the control, cut a small hole in the lower corner of the plastic bag, insert the cable and sheath through the hole, secure the bag to the sheath using a rubber band, then add a 20 ml of light oil to the bag, suspend the bag so the oil slowly migrates down the cable, leave it to drip overnight, in the morning, everything should work nicely. In the event that does not work, you will need to open up the brake drum and see if one of the springs is broken or the pivot corroded.