Started the El Camino the other day with my foot on the brake and suddenly the brake pedal went half way to the floor. Now the Brake warning light is on all the time. checked the fluid level and listened for a vac leak, can't hear any. Any ideas as to what is causing the warning light to stay on?

The brake pressure switch. It is designed to turn on the light any time it senses uneven pressure between the brakes. It is located on the frame a little bit behind the driver's side front wheel. Why it is still keeping the light on, I don't know.

I'll check it out. Thanks for the fast reply! the test should be as simple as removing the wire plug?

Yes, that should make the brake warning light turn off, but I'm not sure why it is on in the first place. Maybe the brakes need to be bled, and be sure to inspect the brake lines for any leaks

Here is what my 81 el camino service manual says about the brake warning switch.
The pressure differential warning switch is designed to
constantly compare front and rear brake pressure from the
master cylinder and energize the warning light on the dash
in the event of a front or rear system failure. The valve and
switch are so designed that the switch will latch in the
"warning" position once a failure has occurred. The only
way the light can be turned off is to repair the failure and
apply a pedal force as required to develop up to 3100 kPa
(450 psi) line pressure.

After checking for leaks, Start the car and pump the pedal to see if it comes up. Then move the car and stop it (in gear and with room)....See if the pedal starts to sink again. Sounds to me like it could be the master cylinder going bad. Electrical issues such as the switch do not cause a soft or sinking pedal.

And good luck to you if a rear brake cylinder is the issue and you have those cursed clips holding them instead of the bolt in type!

X 2 on the master cylinder. Your brake warning light is working properly.