71 Dart brakes
The MC should have very little pressure in it. Other than checking for a good fitting ( new lid / reman res) don't fill past 1/4 inch front the top. Otherwise it's likely the seals on the piston bleeding pressure past them, and into the res. Check this by leaving off the lid pumping up the pedal, and push hard. Have someone watch for bubbles or rising fountains..
DO NOT pump the brakes with the top off,, unless you want brake-fluid all over your paint!!
Brake fluid is excellent paint remover,, rinse with water immediately if you do get any on paint you care about..
If there is no irregularity in the mating suface,, and the seal seem okay,.. I've found that air trapped in the brakes can violently shoot a fountain of br fluid back into the reservoir, up against the rubber gskt,, and migrate under the lid..
With the rear brakes properly adjusted,,, set the emerg brake,, have someone pump the brakes and hold them down,, THEN take the lid off,, and watch the fluid as the pedal is released..
If you have a "fountain" of fluid returned,, especially in the disc reservoir,, then you have air in the brakes that is compressing when applied,, and "springing" fluid back to the reservoir when released...
Poorly adjusted shoe style brakes can cause a similar return,, as the springs return the shoes,, but should be negligable if brakes are adjusted properly..
hope it helps