Truck sat in garage unused and one day fluid was on the ground. My father tracked the leak back to proportioning valve. Tried refilling to find leak but couldn't tell from which lead the leak was coming from. Possible fitting leakage? Maintenance book doesn't show this part for maintenance or repair.
Wash the area, then hold pressure on the brake pedal to see if fluid continues to leak. A fitting didn't start leaking from sitting unless it's real rusty. There are o-rings and seals in the assembly but you don't fix them, you replace the entire assembly. No repair parts are available for those valves. Finding a used one in a salvage yard is acceptable because the failure rate is extremely low.