If there's water in the water-hammer arrestors, they
stop working. Also, they may not drain when you drain
the pipes, depending on whether they're bought devices
with bladders in them (in which case, they're either
not waterlogged, or they need to be replaced), or
built up little vertical stubs with bleeder valves
at the top. In the latter case, drain the pipes
from the lowest fixture available, and open the valves
in the arrestors.
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