If the pedal is firm, the problem is likely not air in the system. Air will give you a mushy or rubbery feel on the pedal. Low pedal with a solid feel can be one of two things, you either have too much free stroke in the pedal and linkage prior to the MC that needs to be taken up prior to actual force being applied to the system, or one or all of the wheels has too much slack in the shoes or pads which needs to be used up before force is applied.
If the problem is at the MC, you may be able to adjust the mechanical linkage or pushrod length to remove the slack. If this started after you replaced the MC, it is also possible that you ended up with an incorrect part. You indicate that the pedal does not pump up which seems to indicate this is where the problem is.
Too much slack at the wheels will give a low but firm pedal, but will pump up to make the pedal higher. Subsequent driving and reapplication of the pedal will bring you back to a low pedal as the fluid will have been pushed back out to the MC via the drum brake springs or from problems with the disc brakes. The drum brakes are easy to sort so long as they are still within serviceable wear limits and the adjusters are not seized. All that is needed is to take out some of the slack with the adjuster to hold the shoes closer to the drums. For the discs, the problem can be excessive runout on the rotors. Runout will cause the pads to be pushed further away from the rotors as they rotate thus causing a low pedal. Thickness variation can have a similar effect, but will give you a pulsing pedal feel which will be the more apparent symptom. To correct rotor issues you will need to either have the rotors machined, or replace them. A sticking caliper or caliper slide may also create the symptom, but usually poor brake performance will also be present.