fluid being drawn through the modulator is a very real possibility. One of the smaller trucks in my dad's tree service "fleet", a 73 f350 dump truck, trashed the valve seals and rings, and transmission, due to this problem over a prolonged period of time. Keep a close eye on it. If the fluid level continues to drop, be sure to check the vacuum line to the modulator for signs of fluid, or replace the modulator outright.