I recently assembled my WC system on my garage floor and had it run for 2 days without incident but i am a little concerned, nonetheless, about my radiator. I forced the water in and thought i had gotten almost all the air out. It ran for about 6 hours near silent and then i came in and started shaking the different components to get the last remaining air out. To my surprise when i shook the rad a large burst of air came out and has been like that ever since. I also shook the rad later on again and got some more bubbles, but not near as many as before. I had my radiaotr sideways and it is pretty big (from a 71 Caprice...sry don't know the exact measurement) so maybe I'm just overreacting since it is my first time doing this, but does this seem normal? Do radiators usually trap air in them like this or not? Also, is there any way to test if your rad is leaking in air? Thanks for all the help guys. I really appreciate it.

Normal,
on those big heater cores, good choice BTW , there is alot of air that gets trapped in the corner of the tanks and in the core itself. Turning it upside down and giving it a little shake will help purge the air...
After it stabilizes are you seeing bubbles or merky coolant.
Air leaks make the coolant look merky. Fish tank like bubbles, are trapped air most of the time.
Sounds like you are being observant and that is a good thing keep your eyes out for drips usually around threaded fittings, and they don't always drip where they leak from.
I have found news print laid underneath, to help look for leaks, works good too.
Above all use common sense...looks like all is well.

agreed

Whew thats a relief.

Yea, its just bubbles. Some strange purplish stuff came out at the very beginning but now its only clear water....and some bubbles. I was able to get rid of alot of them by toying with the F&B. Thanks.

P.S.--Does it affect performance to sit the heatercore on its side or does it not matter? Cuz I think i read somewhere that on its side, it doesn't put as much strain on the pump and u get better flow or something? Any truth to this?

The shorter the distance your pump has to push water vertically, the better your flow rate will be.

Take say a common 350 g.p.h. pressure pump. Most are measured at 0 feet. Raise the level it has to pump to 1 ft. and it drops to approx. 320 or so. Depending on the pump of course.

If you lay the rad on its side, place the inlet tubing low, and the outlet tubing high.

This will promote the expulsion of air.

That strange purplish stuff, was probably just residue from manufacturing, flux from soldering an d such. I would make sure all of that stuff is out, before I closed the system loop.
rinse with hot tap water and from both directions,lots of volume not to much pressure.