Leaking from rad. overflow port after rebuild

In the process of troubleshooting some other problems after rebuilding the 4.0 in a '93 I noticed that when the engine was cold coolant was leaking from the overflow port (line to bottle was off at the time). After the engine warmed up it pretty much stopped. Let everything cool enough to remove and replace the cap with a new one and it did it again. Let it cool down again and used a cooling system pressure gauge attached at the radiator and the pressure never went above 8 psi..

I'm wondering if there is a possibility that the head gasket did not seat or, (god help the machine shop), the block or head have a crack that is closing up when everything comes up to temp. There is no water in the oil and I dont seem to be loosing any water as I havent had to add any after finally getting it full after the rebuild. There is a slight amount of a black sort of film that seems to be forming in the radiator neck but at first i attributed this to all the hoses being new. It doesn't look like oil but now I'm starting to wonder.

Will check the system with the pressure gauge again in the morning to see what it does when totally cool. Any other troubleshooting tips or ideas would be great.

Thanks

Tim

Something isn't right here. The radiator should be full, right to the ring in the neck. If you leave off the hose to the overflow/recovery bottle, THAT'S when it should leak, because the coolant should expand when hot. As the system cools down, it should suck back into the radiator from the bottle. (If the hose is off, it'll suck air back in and your cooling system won't be full.)