Some time ago the brake light in the instrument cluster of my 2003 Silverado 2500HD began to faintly glow while operating the truck. Even after I had the pads and drums replaced, the light continued to glow during driving. Now, the light glows both while driving and while the truck is in PARK and the ignition is off. My mechanic friends and I are stumped. Is the light demonized or is there a rational explanation for its behavior?
You MAY have a master cylinder or booster fault.
Is the brake fluid up to the full mark? No moisture in it?
Check the level of the brake fluid in the reservoir. If this is OK, you may have a faulty fluid level switch in there. Check it by jiggling the float while someone observes the behavior of the dash light.