would spraying it with silocone lube help to unfreeze it?
Do you mean they are binding on the rear disc or is the piston(s) seized in the caliper? If you are suffering from jammed pistons then, having disassembled the caliper, some lube on the outer walls of the piston(s) will help prevent the problem occurring soon – make sure the lube you use will not affect rubber seals – some silicone sprays will. Replace the seals and the bleed nipple. Otherwise avoid any lubrication in the brake assembly (except some copper grease on the reverse of the pads to prevent squealing). Silicone especially is a nightmare to completely remove if you get it on the disc – and will contaminate the pads. Use brake and clutch cleaner it will evaporate and leave no residue.
If your brakes are seized especially on a 79 then most likely just getting them unseized is just half your problem. What I do to unsieze them is take a rubber mallet and give the outside of the caliper a whack. Most likely you are going to have to rebuild the caliper because once you pull the lever again they will seize again. I would check the pistons for any rust or pits if they are they are going to have to be replaced along with the seals. I always find lots of corrosion and old brake fluid gunk so I would recommend having a reliable shop do it because getting those frozen pistons out can be hard and dangerous if you don't know what your doing.
Open the bleeder (with a hose to direct the brake fluid away from the bike) and try to pry the pads apart with a screwdriver. Careful not to scratch up the rotor.
