86 Honda CT500C - Front brakes don't activate brake light

cOULD LEAKING FORK SEALS CAUSE FRONT BRAKES TO FOUL? -- YES
You shuld get a manual, you must check the surface of the fork, if there are scratches, you will destroy a new seal.

Have you checked that its the right wattage for your bike .One way to test you bulb is to attack to the battery and see if the bulb gets any brighter .if it does then you may have a slight earthing problem .check to see if none of the wires have been trapped or are touching the frame anywhere ,

You should have disc brakes??? They are not adjustable.
It is common for a caliper piston to freeze up or become corroded and bind up so it will not move freely.

If your brake lights are not coming when pressing the foot brake this means that there is something wrong with the switch..Have a closer look near the brake lever and you will see a spring attached to it which in turn is connected to the brake switch..Try to pull it down and if you see your brake lights working then you only need to adjust the spring setting..Else try to make a manual close connection and if it goes on then replace the switch..

you may want to trace your wires to that light and check to make sure that your ground wire for that side is clean and tight that may take care of your problem all together