brake warning light

Anytime you work the bleeding brakes on these early AMC's with this brass block/switch assembly in, you should remove the plastic cap, plastic break away rod, spring and brass cup.  What happens and Mike Maher has said above is it is monitoring the front and back brake pressures, when a dissimilar pressure is felt at the pistons inside.....they move to either side (depending on which one is low or high pressure)...they pisons have a small taper which allows the spring loaded plastic pin to drop down and the brass washer under the spring to make contact with the brass housing and provide a ground path for the terminal/wiring connected to the unit.  During bleeding, the pistons have a tendancey to slid back and forth, which is not a problem when the switch is not in place, but when they are in place, the movement can be enough to break off the end of the plastic break away rod.....now is when you have a problem.....the brake will work normally and fine....this an important indication only feature.  I have bought far to many cars over the years with the wire off the switch or some other means to keep the circuit from getting a ground.  If you find the plunger is broken, while you have the switch out, look inside the unit....it is very hard, the hole is small.  But you should be able to see the pistons...you must have them centered...with no brake pressure they "can" move with a pic, becareful no to scratch or damage the pistons....you need to get the broken piece of the plunger out of the hole.  I normally use air pressure.  NOTE: If pistons will not move, you will have to remove from car and disassemble the end caps.....but it is all real easy.  Once removed, recenter pistons, go pump the brakes a bunch of times to make sure the pistons stay centered...if they don't you have something else going on!!   Once you confirm the re-center, then you need to re-install the switch with NEW plunger.  Contact Steve at Muscle Car Research...he is a member of this Forum and he has Switch repair Kits along with full rebuild kits....highest quality and I have used them many times..here is his web address: https://www.musclecarresearch.com/

Is that light also used to indicate the parking brake is applied? If so, it may be an issue with the parking brake (cables not adjusted).

The park brake light switch is on the park brake mechanism......single wire.