If i was you i would strip everything back from the calipor there must be a blockage somewhere slight kink or compression on one of the pipes because that is not right
Get someone to pressure the system and loosen off the pipe that feeds the front Calipor from the ABS Box and see if any fluid comes from there...if there is fluid coming out that must mean that there is a blockage further on it nothing comes out i would take off the ABS Box and stip and clean it and blow out with pressured air system ie tyre blower upper and see if that helps.........the strange thing about this that there is no fluid there at all so that tells me that someone has tried to fix a brake problem and in turn they have damaged ,put dirty brake fluid in so on?
I had a similiar problem on a 93 audi 80 2.0. When I braked the the front drivers caliper cylinder wouldn't return to its home position but the others worked normally. This meant the brake was engaged at all times. As a temporary measure I had to crack open the bleed nipple and relieve the pressure. Once I changed the brake fluid in the system the problem disappeared. If you are doing any work on the ABS unit be very careful. If the slightest bit of dirt gets in and scores the internal valve surfaces you will have to replace them or get a new unit