A little more info would be nice, as to what type of pump and what is its sensor? I think building another sensing mechanism when this should already contain one is not the right approach. It would be better to spend energy troubleshooting the current setup. Is the pumps sensor false triggering, or is it missing a comparator component that tells it the off state compared to the on state (what the PIC should be doing in this case, a voltage comparator would be better).
I think the motors built in sensor is a pressure sensor, or it needs a pressure sensor to complete it (again more info would be great).
Read up on Liquid Level Detecting, might be of interest to you
http://www.freescale.com/files/sensors/doc/app_note/AN1516.pdfI'm a student going into the Biomedical field, (Medical equipment meets Electronics Engineering), we primarily always discuss the significance of sensors and sensor design. Another alternate approach which probably is simple but I would rank second after the above mentioned setup is to create a displacement based sensor, in this case a capacitive sensor. Having a top floating conductive material and an anchored bottom material you create a capacitor with changing distance of plates. This can be used to set a lower bound threshold that is checked every 10 minutes ... if it isn't met, than the pumps are turned on ... they only turn off when that threshold is met again.
Hope it helps,
EDIT: Oish, I should of read Hero999's post more clearly. He mentions exactly what I just repeated like an echo
... but nonetheless a good idea still.