Ahhh, great minds think alike - I pay a lot of attention to brakes and steering components - figuring that if I can steer and stop, not a whole lot of trouble left to get into.
Have heard that sometimes the level indicator in the reservoir can get stuck - giving a false low indication when the fluid level is visibly OK.
It takes me a little over two quarts of fluid to completely flush the system - I tend to err on the side of seeing a good stream of clear fluid coming out of each caliper/wheel cylinder before moving on to the next one. Always amazed at the crap that comes out each time, even with the regular changes.
How are your pads? If the pads are at the end of their service life and you haven't added any fluid during their life - this could be one possibility to explain the low level. Of course, a leak would be another possibility :-)
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.