Brakes "skip" as I press down. Whats goin on?

Yours has manual brakes,no vacuum booster. So any of the three listed come with the push rod and boot.
Will need to bench bleed the master before replacing it.
Be warned, this could turn into a can of worms. Line fitting siezed to line,bleeders at wheel cylinders stuck/siezed or broken. Im fortunate enough to have the tools and been through this enough times,too many to count.
Go around and spray some pb blaster or whatever and give it time to soak and do its job.
A reason to buy a couple good flare nut wrenches.