Board Thread:Questions and Answers/@comment-25523075-20161020092910/@comment-28905095-20161027203143

Sorry for the late reply, but here are some alternate thoughts.

First of, to check the fullness of Dispensers (or other containers) use a Redstone Comparator in subtract mode which you negate afterwards. That is, if the fullness drops beneath a given level, the negated signal will go high and you could trigger your refill mechanism.

However, a better solution would be to recycle the minecart directly back into your system. That is, at the end of the roller coaster add a cactus with a hopper beneath picking up the minecart and recycling it back to the start.

And just for the records, if you have a Dispenser with a Hopper pointing into it, and a chest above the Hopper the Dispenser will always be full if you set it up correctly. The only option then to keep the quantity up, is to add hoppers leading into the chest, with chests above that, and make a chain of chest and hoppers leading into each other.

But still, your best bet is to reuse the minecart from the previous run, and keeping a proper amount of minecarts in the system.