It is a rebuilt master that could have an internal pressure leak due to a bad front piston seal. This will cause the front cavity/rear brakes which apply first to pressurize the rear cavity/front brakes. Press hard on the brake with the lid off while someone is watching with a rag in hand. I have seen this in the past . Very rare but it does happen.
Also the new rubber gaskets on this reproduction masters are not rubber. They a different material and should not be used with Dot. 3
The seals are a different material. They are very shiny and slippery. That also could be the problem. I see yours is square in the front. That is definitely a reproduction not a rebuild .
Read the info below page 3 caption. The newer reproduction master cylinders use a different fluid or they do leak
http://www.moparts.org/Tech/MoparAction/Master_Cylinder.pdf
