Nagayama et al. looked at the effect of adding ascorbic acid (Vitamin C) to a dose of levodopa/carbidopa. They found [1]:

"significant increases in AUC and Cmax, and a significant reduction in Tmax by adding AsA were observed in 25 patients with baseline AUC < or =2500 ng.hour/mL. In conclusion, AsA can improve LD absorption in elderly PD patients with poor LD bioavailability".

Unfortunately, the paper is behind a pay wall and the abstract doesn't give enough information to determine the size of the difference. (Significance here means "statistical significance" - unlikely to happen by chance - it may or may not mean "clinical significance" - i.e. whether the effect would be noticed.) Also, check whether this was a post hoc analysis and, if so, whether it was statistically examined correctly.

Others in this forum have reported benefits coming from taking levodopa with grapefruit juice. Before going down this route check that this is not prohibited by any of the drugs in your regimen.

soccertese, you mentioned that you had not managed to dissolve Sinemet in water. Try dissolving in Vitamin C.

This observation leads nicely to the suggestion that you can alter the duration of a dose by changing the acidity of the stomach by, for instance, drinking water.

Reference:

[1] Clin Neuropharmacol. 2004 Nov-Dec;27(6):270-3.
"The effect of ascorbic acid on the pharmacokinetics of levodopa in elderly patients with Parkinson disease."
Nagayama H1, Hamamoto M, Ueda M, Nito C, Yamaguchi H, Katayama Y.
http://www.ncbi.nlm.nih.gov/pubmed/15613930

John