HondaCivicForum.com - leak down test results, missing coolant

Just got an 86 crx si and it goes through coolant erratically. My friend and I drove the car about 50 miles on the highway after buying it without any overheating problems. We refilled it and went out for about 10-15 minutes in the city and it looked like a lot of the system had evaporated off. I discovered that the overflow tank is leaking badly, so this might be a reason for the missing coolant. I'm still working on getting that out and either patching it up or putting in a replacement.

I performed a leakdown test today and found that my cylinder 4 was getting 90-100% loss at 10 psi, whereas the other 3 cylinders got 40-45% loss at 10 psi. There's no coolant in the oil (as far as I can tell) and no oil in the coolant, nor did I see bubbles coming out of the radiator when I was doing the leakdown on cylinder 4. It doesn't seem like this problem is related to the cooling problem (though I will CERTAINLY address this one).

If it were a leaky head gasket or cracked block, then I should see bubbles in the coolant at the radiator cap, right? Is it possible to lose enough coolant to overheat if my overflow tank is leaky? When I pour water into the overflow tank, I hear water *pour* out, so it's probably a bad leak.