Try this: click your parking brake down one or two clicks and see if the pedal is firmer. If so, it means you need to adjust the rear brakes. This works because the parking brake basically tensions the same thing the adjuster does in the rear drum brakes. If this does not help, does the pedal sink to the floor gradually? If so, the master cylinder is bad. I would also check the rubber lines too as they can become swollen with age and restrict fluid flow. I don't think you need to spend money on upgrades here, you just need to get the stock stuff working right. If you want to upgrade, it's not a bad idea considering the rather inadequate nature of the stock parts when driven hard, but upgrading likely won't fix anything that is wrong unless you inadvertently fix the real problem when you set up the new parts.