This truck probably has Rear Wheel Anti-Lock (RWAL) brakes. There is a small black module near master cylinder that is the brain for this system. There are ways to check for diagnostic trouble codes in this system. Once you have any codes set, diagnosis can be fairly simple.
Diagnostic Trouble Codes (DTC) 's can be displayed by jumping terminals "A" and "H" of the Data Link Connector and observing the flashing of the "BRAKE" warning light. This should only be done when the brake lamp is "ON".
The terminals must be jumped for about 20 seconds before the code will begin to flash. Count the number of short flashes starting from the long flash. Include the long flash as a count. Sometimes the first count sequence will be short. However, subsequent flashes will be accurate. If there is more than one failure, only the first recognized code will be retained and flashed.
DTC's 6, 9 and 10 will only turn the "BRAKE" lamp on while the fault exists. These are soft codes faults and can only be read with the Tech 1 scan tool.
Do not jump terminals "A" to "H" if the "BRAKE" lamp is not on. Jumping the terminals with the "BRAKE" lamp off will set a false DTC 9. This will turn the "BRAKE" lamp on and disable the antilock system.
1, 11, & 12 ECU Malfunction
2 Open Isolation Valve or Malfunctioning ECU
3 Open Dump Valve or ECU Malfunction
4 Grounded Antilock Valve Reset Switch
5 Eccessive Actuations of the Dump Valve
6 Erratic Speed Signal
7 Shorted Isolation Valve or Faulty ECU
8 Shorted Dump Valve or Faulty ECU
9 Open Circuit to Speed Signal
10 Brake Lamp Switch Circuit
13, 14 and 15 ECU Malfunction