Master Cylinder - Cover Leaks

You likely still have air in the brakes. When you push the pedal, the air in the system compresses, thus the poor pedal. When you release the pedal, depending on the amount of air, a small burp, up to a large violent fountain of fluid can be propelled back into the M/C. The cap isn't designed to contain fluid shot at it. thus fluid migrates under the seal..

You can verify by having someone pump the brake pedal a few times WITH THE CAP ON, have the pedal held down, remove the cap (but keep it fairly close over to control the returned fluid) and have the pedal released. Whichever system shoots fluid back, (front, rear, or both) needs bleeding..

CAUTION.. Brake fluid is excellent paint remover and needs to be rinsed of any paintwork IMMEDEIATELY with lotsa water ! !

I often wet the whole area with water to prevent paint damage by any fluid spray..

hope it helps..cheers