The valve parts are plastic and/or rubber. Laquer thinner will probable cause some trouble.

You need a new switchover valve. Not too expensive, I think. The plastic cap is factory, it keeps crud out.

You may be able to ream the hole out, but don't count on it. Try some brake parts cleaner, and blow backwards through it. If it clears, you're good to go, at least for a while.

Switch grounds the switchover valve, its supplied with voltage all the time.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!