eeek... dont use a high capacity air source for sure... hitting it with air from your typical gas station set up will risk blowing seals...

also it should be 'atmostpheric" while the rear wheel is in the air so that when the weight of the bike is on it, it will be pressurized a few lbs under normal conditions. If you do vent it while the wheel is on the ground and the shock is slightly under pressure, then you have 0 lbs pressure under normal conditions.

I used a foot pump for a bicycle and gave each one 5 equal pumps.
Then I checked it with a tire gauge...and of course...lost all the pressure and couldnt tell anything...LOL.

So I bled all the pressure from both and then repeated the 5 pumps each ....it rides better now...

Gadget has some great tips on doing this right... http://www.gadgetjq.com/airshockbalance.htm

 

What you need is a "zero loss" air pump. The mountain bike fellows use them for the air forks on their bikes. Come complete with an air gauge, too! Available at your local bicycle shop.