>have installed a sewer line in a mojular home and would like to know the drop in inches to the septic line.<
The minimum slope in the sewer pipe is defined in the plumbing code in force in your specific jurisdiction. Generally though, most codes require at least a 1/8 inch drop per foot of 3-inch or, 4-inch sewer pipe. So, that is 1-inch of drop for every 8-feet of run. Check the local code and install the pipeline to the specifications defined therein.
>Should the line hold water or be dry when water is put through it.<
The purpose of a minimum slope in a gravity sewer pipeline is so that water will not remain in the pipe.
>I have a leak and water drips most of the time I know I need to re plumb but not sure how much drop in inches in the line.<
If you are using PVC, or ABS plastic DWV (Drain, Waste, and Vent) pipe, be sure to clean thoroughly all glue surfaces with the appropriate solvent/cleaner, and follow precisely the instructions on the container of cement. Attention to detail will prevent leaks in the future.
>about 24′ from 1st hook up to main septic line<
Seems like the MINIMUM fall in the sewer pipeline is 3″. The slope of the finished pipeline could be, and probably will be, greater than 1/8 inch per foot (up to vertical), but it can not be less than this grade. Again, check the local plumbing code to see what slope is required, it may be 1/4 inch per foot in some jurisdictions.