Power Disc Brakes not working on hard stop?

Power brake boosters have an adjustable plunger to the MC if it is set with too much gap you will have to little pedal travel to fully move the mc pistons.

( too little can move the mc piston in a bit and cause it to block ports that are needed for normal operation)

Someone else made the point of the mc bore diameter. Smaller diameter means less leg pressure but more pedal travel, larger diameter is less pedal travel but more leg effort.

With power brakes you don't need less leg effort so you should have the OEM size or larger.