Two weeks ago, the brake pedal started sinking all the way in once I pushed it but once I let it go and push it again it would start working back normally(hard and firm) it'll work fine for about 3-5 minutes then it sinks again.then it repeats. It'll start working normally. I replaced front calipers and brakes two days ago but it still does the same thing. What can it be? Rear brakes? Brake booster? It's a 1997 xj 4x4 with 172k. Thanks!

 

Most likely your brake master cylinder is bypassing.

Have you adjusted your rear brake shoes?

 

^^
What he said.
If you don't have air in the system, it's the master cylinder.