Use pedal effort, vacuum behavior, and code context to separate a failing brake booster from intake leaks, check-valve trouble, pump problems, and biased booster-pressure sensing.
If pedal effort is obviously higher than normal, treat that as real evidence of lost assist even before deciding whether P0556 or P0557 is electrical or pneumatic. The code is there to help sort the branch, but the hard-pedal complaint is what tells you the brake-support system may truly be underperforming.
A hard pedal during repeated stops, low idle vacuum, a split booster hose, failed check valve, or weak electric vacuum pump all fit the reduced-assist story better than a random sensor replacement. If assist improves briefly after startup and then fades, vacuum supply and storage become especially important.
The brake booster lives on the vacuum side of the engine on many vehicles. If P0556 or P0557 travels with lean trims, rough idle, or a hiss near the booster hose, do not be surprised if the booster circuit is leaking badly enough to affect both braking feel and fuel-trim behavior.
A biased pressure sensor can accuse the system even when pedal feel is still mostly normal. In that case the scan data, measured vacuum, and actual assist level disagree. That split matters because replacing the booster for a clean electrical fault is just as wasteful as replacing the sensor when the pedal is genuinely hard.
First verify whether pedal effort is truly abnormal. Second inspect booster hose routing, check-valve direction, and vacuum supply or pump operation. Third compare any available booster-pressure data with real vacuum and complaint timing. Fourth only decide between booster, hose, check valve, pump, or sensor once those stories either line up or contradict each other clearly.