I used brake parts cleaner on mine. It blew out a bunch of black gunk. My banjo bolt was filled with gunk as well and I used the same process to clean it. Sure runs better when that stuff is clean!

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2005 Accord Hybrid (Wifes)
1995 Subaru Impreza L AWD (Snow car)
1984 GMC Sierra 1500 (Mine)
1983 300CD Best $ I ever spent. (Mine)
1984 190D (sold and glad I did)
1983 300D (sold and wished I hadn't)