I have 1979 chevy nova. Recently the warning light comes on, sometimes dim and sometime bright. Sometimes, push on brake pedal and it goes off. There is no brake fluid leak anywhere. When u push on emergency brake light comes on bright and goes off or gets dim when released. Unplugged brake pressure switch and light goes off. Plug back up and it is dim or blinks. How do u test pressure switch. Brake fluid is full and no leaks. What can I test or do???? Thanks
Your brake warning light serves 2 purposes It tells you when the parking brake is on and it warns you of a difference in pressure between the 2 hydraulic brake circuits.
The light has power at all times "Key ON"
Closing either of the switches independently, either the Parking Brake Switch (by pushing down the parking brake pedal), or Closing the Pressure Differential Switch (mounted on the combination valve) when the switch's pin is pushed up by the spool valve,
will provide ground to the warning light, complete the circuit, and illuminate the bulb.
You can test the Pressure Differential Switch by removing it (reconnect the switch plug ).
Pushing the pin in on the switch will illuminate the Brake Warning light . You may want to shake and smack the switch with the pin all the way extended to see if jostling it causes an internal short in the switch and illuminates the Brake Warning Light when it shouldn't. (faulty switch)
((if only one wire is going to it be sure to ground the metal on the switch when you test it))
The thing is, it's a $15 part. If you have the old one out to test it, you may just want to install a new one instead of putting the old one back in.
The switch pin is pushed up by the Combination Valve spool when unequal pressure exists.
It looks like this.
Brake Warning Light OFF (equal pressure)
Brake Warning Light ON (unequal pressure)
So if it isn't a faulty switch or air/blockage in one of the brake circuits you can try a get the spool to "recenter" itself by slamming on the brakes at a slow roll (mimicking an emergency stop) a few times.
Sometimes it works. Sometimes adjusting the rear brake shoes closer to the drums and repeating the "hard braking" works.
Of course any air in the lines has to be bled out.
Sometimes the Combination Valve needs to be replaced. (It isn't expensive)
Here's a link on how to disassemble your combination valve if you want to see all the components.
https://www.musclecarresearch.com/content/1979-gm-kelsey-hayes-combination-valve-autopsy
Check for frayed wiring or a bad ground connection. Check wiring to master cylinder float for loose connections or disconnects.
