every recipe for liquid sinemet i've read suggests dissolving the tablet in orange juice or adding vitamin C to inhibit oxidation.

my apologies if you are already doing this.

here's a recipe i found

grind up ten 25/100mg tabs plus 2000mg vitamin C
add 1000ml water, juice or soda pop
concentration is 1mg/ml., should keep 2-3 days, refrigerate. discard if it turns dark.

you may have to take the liquid sinemet more often than the tablet since it goes thru your system a little quicker. i assume this means less is absorbed thru the small intestine? just guessing.

page 257 in "the parkinson's disease treatment book"