When sliding stanchions to lower legs , should I pump up air to fully extend the fork before tightening bottom nuts ? Read somewhere that if not done it will create vacuum in lowers which affects fork performance. Does it really matter ? I would assume slight vacuum could actually help to make breakout force lower ?Fox service manual does not mention it..

Should be done fully extended otherwise you will cause a vacuum in the lowers.

It will act like a negative air spring however the effect will be short lived as the air pressure will eventually equalise.

PS Tried it on my Fox36s, it’s a pity they don’t sell a range of negative coil springs, soft, medium and firm.