If there's no accumulation of brake fluid beneath the modulator and reservoir is full, then modulator plug leaks is not your problem. Other possibilities are pump malfunction (check valve leak or leaking pump piston seals) or internal modulator valve leak. Unfortunately, these are not repairable to my knowledge.

Good used modulators can usually be purchased for <$100 if you shop around. Check ebay and car-part.com for units near your location.

good luck