Diagnosis
The unloader valve assembly has internal seals that direct high-pressure water. A leak from the body means those seals are worn or damaged, or the valve body itself has a crack (less common).
How to Fix It
You need to rebuild or replace the unloader valve. First, relieve all pressure and disconnect the spark plug. The unloader is the brass block where the high-pressure hose connects to the pump. It's held on by several bolts. Remove it. You can often find a rebuild kit (like a "gun/unloader seal kit") that contains the specific O-rings and seals. Disassemble the valve carefully, noting the order of the springs, plungers, and seals. Replace all seals with the new ones from the kit, lubricating them with a tiny bit of silicone grease before reassembly. Tighten the body bolts evenly.
If That Didn't Work
If it still leaks from the same spot after a rebuild, the metal valve body may have become scored or cracked from freeze damage or grit. In that case, you need to replace the entire unloader valve assembly.
Heads Up
When you reassemble, be very careful not to overtighten the bolts on the brass body. They only need to be snug. Over-tightening can warp or crack the housing. Also, always run the machine for a minute with the trigger gun pointed in a safe direction after any repair to check for leaks before you start working.