well. It charges the system to 50pounds.

if it chgs the sys to 50psi then wouldn't that mean your low psi on the shower
would be further down stream? Have you emptied out the water in your tank and
checked/ added air if needed ? the rebuild kit is only for a failed mechanical/
shaft seal. If pump isn't losing prime or leaking water at the shaft then no
need for kit. if that shower is only one w/ psi prob i would check pipes close
to the valves for rocks or other crap.