Thanks for the update. As it turns out, I have a bad head gasket as well.

I ran the same test (basically a leak-down test but without the leak down tester) by using a compression tester's spark plug fitting (rented from AutoZone) and hooking it up to an air compressor set to 30 psi (bought a small one at WalMart for $50, needed/wanted one anyway). I listened inside the radiator cap opening and heard bubbling/air flow through the 1 and 2 cylinders Ironically the same exact cylinders as the OP.

Not sure if i want to try Steel Seal or just replace the head gasket over the weekend...kind of afraid of gunking up the system with that stuff...

For other people doing research on coolant leaks, here is what happened in my specific case and what I did to diagnose it:

I have a 2004 Civic LX stick shift, and a few weeks ago my car overheated because of what turned out to be a bad seal between the radiator cap and radiator. I replaced the radiator cap but it didn't do the trick because it was actually the inside of the radiator neck that was corroded away (which also meant that there was probably corrosion and deposit buildup inside the radiator that I couldn't see). So I got a new ReadyRad radiator at AutoZone and installed it, and flushed the system while I was at it. I also replaced the thermostat as it takes about 5 min total on my car.

Once refilled I was driving the car around for another week without any problems so I thought that was the end of it, but unfortunately on a really hot day last week the car overheated again. This time the loss of coolant was due to it boiling and being pushed out of the radiator into the overflow reservior. I did a pressure test and found no leaks so I thought it might just be air in the system. Since my Civic doesn't have a bleeder valve I parked it on a steep hill (tire backed into the curb so I could keep it running in neutral), put a funnel into the radiator neck and let the car run for a while until the thermostat opened a couple times and I assumed most of the air was out. Again I drove around for a couple days and it seemed fine but sure enough I noticed the needle start to go up above the half way point while driving and found coolant was boiling into the overflow again.

At this point I started thinking it was the head gasket but a few signs made it questionable:
-no white smoke or sweet smell out of the tail pipe.
-no bubbles in the radiator neck when first starting the car and reving it to 3000 rpms.
-no yelow fluid in block test (described below)
-good/consistent compression (described below)

I did a compression test (rented from AutoZone) and found that all four cylinders have about 110 psi, which I've read isn't abnormally low for this car. I also did a block test (also rented from AutoZone...had to buy the blue fluid for $7.99) where you drain the coolant just a couple inches and stick a tube into the radiator neck and fill it to the line with blue fluid. Supposedly, if there is exhaust getting into your coolant system it will find it's way to the top of the radiator and get sucked into the tube and the blue fluid will turn yellow, then once you remove it the fluid should turn blue again (otherwise you may have sucked up some coolant giving false results). I say "supposedly" because my fluid stayed blue the whole time as my head gasket leak was probably too small for the tester to pick up on. I even reved the engine a little but never got the fluid to turn yellow. As a sanity check I held the tester to the tail pipe and the fluid turned yellow within 30 seconds and then back to blue once I sucked in some fresh air.

I also checked the coolant circulation while reving the engine to make sure the waterpump is good. I tested the fans by shorting the harness that connects to the radiator fan switch (simulating hot coolant that would trigger the fans). I tested the radiator fan switch (side of thermostat housing), coolant temperature sensor (top of thermostat housing), and thermostat using boiling water. Everything looked good...

Finally, I did the makeshift leak-down test, which proved that I have a blown head gasket.

In hind sight, I should have done the leak-down test first but I only ended up getting to that point by process of elimination...partly because I was hoping that it wasn't the head gasket...oh well.

Hopefully this info will be useful to others since it was driving me crazy when I couldn't figure out what was going on.

 

Actually, this is a good example of why you need to repair coolant leaks immediately. The pressure that builds in the system is important to prevent the coolant from cavitating. (if you have a leak, the pressure doesn't build). If you get cavitation, you get hot spots, and a risk of warping/headgasket damage. Obviously, if you let enough coolant out to completely overheat the engine, you really are asking to scrap the block.