Teflon tape or teflon dope is not a sealant! It is a lubricant used to reduce friction in a tapered thread joint, allowing a higher degree of tapered contact surface., which makes for a better seal. You probably have a leak because the tapered threads are mis-threaded on one or both of the fittings involved.

If you want a seal, use Expando. You will never get it apart again, but you will never have a leak again either. Expando can be purchased at any industrial fitting supply house.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport