Most hydraulic valves, called "Spool Valves",
because they look like old time thread spools,are
precision machined, and then "Lapped in" to a precision fit. Spools are not normally interchange
-able in any one valve body, or even between the same make and model of valve.
(There are some exceptions now, where the spools,
and bodies are machined with such precision, that
they are not lapped)Valves hold hydraulic pressure
by the metal to metal fit. There are no O-rings in
everyday valves,except at the ends of the spools,
to prevent external leakage, If your system is
"leaking down" it may be that the valve is worn out, and must be replaced,(I know of no maker
that sells replacement spools), or more likely
the cylinder seals, or packings, are worn out from
back and forth motion. (Seal wear avoids metal to
metal wear, in cylinders, which is much more
expensive to repair) Another note: Hydraulic
motors, (Think: Char-Lynn) have a definite "Life"
and can have internal leakage.
Your valve has no "Missing O-Rings", but is likely leaking down due to internal wear!
On Giant excavators, and power shovels, such as used in open pit coal mines--where the buckets are large enough to hold a pickup truck, the valves weigh several hundred pounds, and cost
thousands of dollars, and are indeed rebuilt, by the "Spray Welding" process, and remachined,
usually they are returned to the factory for that.