I am trying to get my old 56 gmc with 235 road worthy, but the radiator keeps periodically blowing the anti-freeze out the cap. I had the radiator re-cored last year, and put in new thermostat 2 years ago. The truck seems to run fine, and warms up okay,when I start it up in the driveway. But then once in a while, boom, the pressure blows through the cap, emptying half the water. Could this be an intermitent thermostat issue, or something else.
