Iron or aluminum heads? I did a set of heads to replace a set of 915s that only leaked when hot. Pressure checking doesnt reveal everything.
If Iron heads: It's fairly comon to have a coolant leak from under a replaced seat, if they cut too deep. Have the heads completely stripped, and magnaflux them. Pay attention to the inside of the exh ports. Any rust in any of the ports?
If Aluminum: Take them completely apart, have them put in the pressure checker, and pressurize them to 50-60 psi. That should be high enough to start a seep if there is a crack. I found many shops that only test to like 15-20psi, because "that's where they run at...". A higher test pressure will force things out more. Again, pay close attention to the seat areas and guide ares. It wouldnt surprise me if you have a leaking seat or crack in #3. Especially if it was bad enough to hydraulic it.
