Does the rubber cover over the modulator adjustment hold the vacuum? When mine is off it does not hold vacuum. Now it's all dried out and split. Tried another off the kids wreck but it does not hold 100% either. Are there new ones available?

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)