Well top it off and see how it drives just a short distance to see how the pedal reacts....though you may have sucked in air in the small part of the reservoir due to the low fluid, hopefully the master cylinder isn't fubar....so re-bleeding might be in order.
I would also visually inspect front flex lines, fittings, calipers, non adjustable proportioning valve usually located directly below the brake booster near the frame rail where the brake lines converge coming up to the master cylinder and look for fliud leaks.
Check the rear backing plate to axle side underneath where the adjustment slot is to see if there is a fluid leak, wheel cylinder/s will bleed out that side so you'd have to pull off the drum/s and make repairs, replace both wheel cylinder/s if the seal/s are gone....PITA tear down job but wheel cylinders are not expensive so get a good quality brands.
With the drum/s off, check each backing plate "bolts" (4) to axle and make sure they're torqued correctly, not loose otherwise when you brake backwards/forwards the backing plate will "ratchet" causing the brakes to lock up with the slightest touch on the pedal and possibly snap/shear off a bolt and really be in the shite not too mention the carnage inside.
If for some reason you dodged the bullit with low fluid in the reservoir and the pedal comes back, adjust the rear brakes by expanding the shoes out until you can't turn the wheel then retract the shoes until you hear/feel a slight "drag" from shoe to drum. I lift the BKO from the center off the differential on a level floor so it won't go anywhere, using a floor jack lift just enough to get the wheels off the floor a couple of inches max, key on in neutral with parking brake off you should be good to go.
Let us know what you discover!
Good Luck ~ :thumbup
