it's a mechanical force system. very simple in theory.
if your caliper ratio is wrong 'enough', you can end up with very little lever motion, and not enough force on the calipers (too large of an MC caliper), and if you have much too small of a hydraulic displacement, you'll end up with a very long throw on the lever to generate force, but you'll be able to generate more force.
in either case however, you'll be looking at a linear force - you won't ever 'build' pressure.
I recommend a pressure bleeding. sink a line into a bell jar with brake fluid in it, open your MC and keep it full of fluid and pump the whole system out until the tiny bubbles stop coming out.
Possible causes for this problem are air in your lines, compromised brake hoses or water in your brake fluid (which can cause elasticity)
if your lever feels 'springy' at all instead of firm, you've got some compromise of the hydraulic system