line flies out of troy-bilt trimmer WAY TOO FAST!

Is there a fix or work-around for this problem? Our weedwhacker, a gas powered Troy-Bilt (like the Ryobi) used to let the line out properly (when we bumped it on the ground) but now, it lets itself out - it's always at the max length and chops itself off - I never bump it and it runs out of line very fast. we had the bump knob off and cleaned the assembly, did not see any obvious problem except the foam seal is a little bit torn.
Help? Jane
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