You need to check all brake lines and look for leakage. Also you should check the main brake cylinder for leaks (you can find under the hood attached to firewall on the driver's side).
Check the brake fluid reservoir, it may be low on brake fluid indicating it is time to have the brakes inspected.
check brake fluid level under hood probaly low
it may need some brake fluid
Light on dash? Check fluid level in master cylinder
The Parking Brake could be set (even slightly)
Brake Fluid.
Check the brake fluid level in the reservoir there is a sensor there that will make the light come on if your level is low.
I have noticed that if the brake fluid is low enough in the brake fluid master cylinder it will cause the light to come on when turning left. before it comes on for a low fluid problem because of the fluid shifting.
In troubleshooting a Chevy S-10 brakes, first check any leaks by the wheel. It may come from the wheel cylinder or from the caliper. Also check the brake fluid pipes for possible leaks that affect the brakes.
Check the brake fluid reservoir. If it is low, the light will come on notifying you that the brakes need to be serviced.
Could be low on brake fluid. This is a sign that the brakes need attention.
Open brake fluid reservoir for the brakes - make sure it is full of brake fluid - put lid back on reservoir then pump the brakes then hold the brakes on - have someone open the bleed screw on the brakes and let the air and oil out and close it again quickly and repeat until the air bubbles stop in the brake fluid, also checking to make sure you do not run out of brake fluid before you finish bleeding the brakes.