I replaced my front brakes a while ago. The brakes had about 40k, so they did have to be replaced. While removing the caliper bolt, I pulled out one of the rubber slides - NOT the boot. Every other time I've changed the brakes, that slide never came out. I've always used plenty of grease on the changes, and the caliper bolts came out with some grease still on them. I've also never replaced the rubber components.
Anyway, since I was able to put the old rubber slide back in by pushing it in with a screwdriver, my brakes have been pretty squishy. I had the brake fluid replaced as well, and that didn't improve it.
Would that rubber slide make that much of a difference? A new set of rubber components is pretty cheap, so I'll buy them and spend an hour swapping them if that will fix it.