to vent or not to vent ?

hi all,
i fitted a power-shower a few months ago (in my own home) and it keeps getting air-locked in the above pipes. both hot and cold feed are gravity fed through the header tank in the loft (hot supply goes through the hot storage tank obviously). both pipes go into the loft, level with the bottom of the header tank, run for only about 1 metre then drop down into the shower below.
i just went out and bought some pipe and tee's planning on fitting vents to both pipes to allow the air to escape but now i'm not sure if this is a good idea. is the shower likely to suck air in through the vents rather than pull the water through the pipes ? there's only about 30cm water head above the pipes in the loft.
what do you all think - vent or not ?
BTW - i presume the air is getting in when the bath has been run and the header tank is empty (i have also bought another tank)
TIA
Chris
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