Mick - balancing carbs is about getting the slides to all lift at the same time - its done at idle speed, you are not lifting the slides in the bore with the throttle so there should be no issue with air-leaks around the slides.
There should be adjusters between the carbie bodies for balancing the slides - you should not have to take the top caps off at all.
Normally you set the adjuster on the 2-3 bodies (in the middle) first, then the adjuster between the 1-2 bodies (on the left) and finally the adjuster between the 3-4 bodies (on the right). Sometimes you may have to go back to the 2-3 adjuster to get the balance *perfect*.
I always use a small fuel tank hooked up on a stand to feed the carbs - fuel tank is taken off the bike so you get access to the adjusters. Use a big fan to keep air blowing over the motor/through the radiator, so it doesn't overheat.
