Yes, the modulator cap is enough to seal it off. Mine was the same way, I replaced the cap and it held vacuum for longer than I desired to wait.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k