Low shower water pressure immediately after replacing a cartridge is usually caused by loose mineral debris, pipe scale, or Teflon tape clogging the internal pressure-balancing spool or the showerhead aerator. When a new cartridge is inserted into an older rough-in valve, accumulated calcium flakes inside the plumbing lines often break loose and migrate forward, blocking water flow. To fix this, shut off the water, remove the showerhead and cartridge, flush the lines thoroughly by opening the valve briefly, and clean out any trapped debris.
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