DIYbanter - auto air vents

i had a play with the eberspacher water heater in my motorhome yesterday as
my calorifier (hot water cylinder) arrived at last, and i found out just
what a bugger of a job it is to get all the air out of a closed loop system,

how i plumbed it was.... boiler located in an underfloor locker hot
water output goes up through the floor, it went to the top connection of the
coil in the tank, i put a T piece connector on that join, and ran a pipe to
a car's expansion tank raised above the level of the coil in the tank,
output from the calorifier coil goes down through the floor to the input of
the circulation pump, output of the pump goes straight accross to the
boiler.

Filled the system slowly through the expansion tank, turned the pump on
manualy and tried all i could to burp the system, wouldent have it,
disconnected the expansion tank and left the tee piece in place pointing up,
dribbled water in there with the pump running untill it overflowed,
re-connected the tank,

still lots of air in the system,

tried the same with the pump not running, same result.

i got the system working by using a bucket, splitting the pipe between the
output of the boiler and input of the calorifier coil, filled it with water
and let the pump suck the water down, wouldent at first, wont self prime it
seems... air lock at the pump, so i ran the hose pipe at full pressure down
the suction line and that got the water flowing,

Then for about 5 minutes it was burping air bubbles out of the feed line
with the pump running.

I gather i'll need to put an air bleeder at a low point on the system, but
was wondering if these automatic air bleeders/seperators would work in this
situation? or do they have to be above the water line like what the
expansion tank is supposed to do,
or do i need to run the system with the output of the boiler coing into a
tank, and suction line taking water out of that all the time?

Or any other solutions,

I was only running about 3 meters of pipe and what ever length the coil in
the calorifier is in this experement, when i get the radiators i'll have
about 20 meters of pipe and 3 rads, maybe that'll help as each rad will have
an air bleeder on it, but i still need to get the air out of the pump to
start with.