A very common reason for Shutoff valve leaks on new REPRO valves and rebuilt originals is small rust particles in gas tank.
The small rust particles get into the valve and score the surface of brass pieces of valve.
Grooves get worn in brass and leaks are result.
After some time the valves freeze up and will not turn.
1. A fix is to install a "finger filter" in the shutoff valve.
https://modelastore.com/index.php?ro...&category_id=0
2. Another thing to do is to dis-assemble
THE NEW valves and grease with
EZ Turn Fuel proof grease from Aircraft Spruce Inc. The grease blocks the rust from getting into the valve.
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I have used this grease on every valve I install for about 15 years.
Note: The valves I have used were bought from Berts Model A store.