Answer did ya' bleed 'em? there may be air in the lines.
Be sure the new master matches the old master exactly and bench bleed the master before you install it.
Its a bad cylinder spring
Most likely brake master cylinder.
Master Cylinder seals are defective. Replace the Master Cylinder ASAP. Stop driving the vehicle until you have this repaired. The brakes can fail completely at any moment.
Wheel cylinder (drum brakes) or caliber (disc brakes) sticking. If you find this to be the case you need to replace both sides and replace your brake fluid which is contaminated with moisture.
May be the proportioning valve by the master cylinder
Bad fluid in master cylinder,flush system,bleed brakes
make sure it's not the rear brakes hanging up, see if they feel hot, if you have drum brakes on the back a wheel cylinder could be hanging up.
the rear brakes are grabbing. the linings may have fluid on them from wheel cylinder leak or dust buildup. sanding the shoes may help .
probably not your engine, sounds like its time for new brake pads. if this doesnt correct it have the master brake cylinder checked
the brakes or the emrgen see brakes
If your power brake booster (the large roundish metal thing that the master cylinder is bolted to) either fails or loses its source for vacuum then your brake pedal will become very difficult to push like if you have manual brakes.
Simply put, friction. If your brakes are overheating then you may have a caliber or wheel cylinder sticking. Replace the part that is sticking and replace all the brake fluid as it is more than likely contaminated with moisture which cause the parts to rust.