Brakes are a very simple system to troubleshoot. In a Nissan Frontier, he brakes start at the master cylinder. When the brake pedal is pressed, the master cylinder sends hydraulic fluid with the help of a brake booster through the brake lines to the caliper. The hydraulic pressure created is what pushes the caliper to the rotor. When you release the pedal, the calipers retracts, letting the rotors spin freely.
Inspect your brake pedal for proper free-play and function. Your brake should engage less than a quarter of the way down. Suppose your pedal appears to have to much free play you will want to examine your master cylinder for any leaks.
Open your hood and inspect the master cylinder. Pop the cap on the master cylinder to check for fluid level. The fluid should be around the fill line. Make sure the fluid is clear. Any color in the brake fluid will indicate the fluid is dirty and should be changed.