Do you have a master cylinder on the throttle perch?
The older airheads had the master cylinder located under the tank, with a cable operating the piston.
If this is what you have, it is possible that the cable is binding.
If you have a MC on the handlebar, with a hose going either to a junction or straight to the caliper, then it might be kinked, especially in the former situation. Where is your brake light switch located?