I read in this post about U needing a tubing line under the thermostat housing. If it is the one that is connected to the Water Pump housing it is what is called a pump vent.

The tubing is steel and it has banjo bolts on both ends. It is about 3 inches long.

Well I got a new one from Phil about 3 months ago as mine had a tiny leak.

When I pulled to old one off the tubing was real thin in an area you could not see.

Glad I did mine. It cost like $13 or $30 bucks I forget but worth it.

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1982 300SD