OLd grease fittings

I am cleaning of neglected grease fittings on a 190Db Ponton Mercedes. Several of the rubber protective covers are really "tarred" in place . What
is a good solvent to get them clean and open without destroying the rubber? This is a 1959 car and if loaded with grease fittings 26 in all at last count, wolf1
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Try mineral spirit that you can buy at Home Depot... pretty sure it will not harm rubber.
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