Start with old school maintenance routine what type fluid? DOT 5 OVER time will do that. It is the worse as absorbs moisture.
But all fluid can. So...
Last was last time brakes were treated to a good bleeding? Manually adjust by you in rear if auto ones have not been doi g a good job.
If calipers are no leak? Sticky ? If no to both you could still just get kits to do a look see. Clean, look see, and hone, clean, assemble.
Some new parts are trash. I try to always refresh the ones I have now. (Ie) May be better and just do the needed maintenance or quick rebuild.
Hey ya did it back when nothing was available, or to poor to get new! And with new stuff being junk to me it's the same [i][/i]as none offered. Maybe worse as you don't want to take better parts off than you are putting on.
I rebuilt my calipers, but found NOS wheel cylinders, old stock master cylinder kit, and has been a long time but new rubber for booster and tested check valve.
Could check and if need be Booster Dewy (so?)
I am kinda an idiot but bought extra drums and rotors when i saw things getting out sourced. But you can find them good USA Raybestos, Bendix, etc good to get just to have and pads, shoes they don't eat food they just sit and wait.