replace the calipers too.
and the rubber lines.
the fact that you were able to add a quart of anything to the brake reservoir means it was probably sucking air into the cylinder at that time.
and the fact it was so low means either the grommets on the reservoir going to the cylinder were badly leaking or one of the calipers is leaking.
do the following:
remove all calipers
drain all fluid
remove cylinder
blow out all lines with compressed air
install new cylnder
replace rubber lines all around
install new (rebuilt) calipers
i doubt that trans fluid in the abs unit would cause it to fail, as auto trans valve bodies are pretty similar. but you will have to bleed it REALLY WELL to make sure there isnt any left in there.
invest in speed bleeders, and bleed one caliper a day for about a month.
if your brakes are still boned after new cyl and calipers you will need to find a replacement ABS unit, but hopefully all those parts will fix the problem.
you may not need to replace all the calipers, but it would be a good idea to rebuild them and do a very thorough check with a scope on the inside checking for any sign of corrosion around the piston area.
Alusil, Dinan DME | ITG Air Filter | Eibach/Bilstein HD | 26/20 Swaybars | Iridium plugs | Depo/ProLumen HID | Optima batt. | no AC | Stoptech brake lines, Frozen rotors, brass bushings, Superblue