If it failed the gas test the third time it's slowly getting worse( just had similar experience with one of my diesels ) unlike the diesel though, internal pressures are a bunch less on that thing so you may luck out with a sealer. the only one worth mentioning is the K&W block sealer. I've actually had a number of vehicles with minor gasket leakage/small cracks get a long term fix with that stuff, the other two products are short term radiator leak fixes.

Follow the directions to a T. get all the antifreeze out before hand and after, get all the mixture out that you can ( remove hoses, blow through etc, heater core too ) get it all out and make sure that you allow the full 24 hour dry time( give it two days if you can )
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale