Head gasket problem
 

Hey guys, I just finished doing the Head gaskets on my ranger 4.0 OHV. I put on brand new heads, the old ones were cracked because the previous owner ran the truck hot with the radiator leaking. New head bolts and tapped the bolt holes and sucked them out, used a felpro gasket. I cleaned the block with a scotch bright roloc pad, and replaced all of the normal stuff like thermostat, water pump, radiator. Everything is torqued right, made marks and torqued bolts to the T. Truck ran good for 15 minutes and then I noticed around the corners of both heads water was seeping out. Everything was done right so now I'm stuck on what to do unless I had some low spots on the block. Any ideas? Vacuumed the system down and pressured tested it before running it and no issues