Hello, I have a 90 Nissan pickup and recently the brake warning light came on. Its full of fluid and the emergency brake is not on. What could cause this problem?
You might want to look at the Load Sensing Valve at the rear of the truck.
On top of the rear differential you’ll see where the brake line enters the valve and then splits off to each rear wheel. The valve will have a rod that attaches the the bottom of the bed. This a crude version of rear ABS. Check and see if the rod to the bottom of the bed has broken off.