03 suburban soft brakes

for the last couple of years this truck has had soft brakes. the pads are fine, fluid looks good. 150K on the truck. could it be the vacuum booster? i bled the brakes a while back but it didn't seem to change anything.
any thoughts?

Is yours drum rears or disc rears? If drums, maybe your automatic adjusters aren't working, you might want to manually adjust those.