[Vwdiesel] coolant burping vw pickup

[Will Taygan]

1980 diesel pickup 1.5L NA.  New head gasket. rebuilt head.

all right, I've filled the engine and the radiator from the top radiator hose with 50/50 phosphate-free coolant and water.  The thermostat has a 1/8" hole in it as well. It runs fine, radiator gets hot, fan turns on, I'm running it with the expansion cap off to burp it.  It doesn't overheat according to the temp gauge.

after 5 minutes or so I get a violent boiling from the main line leading up from the heater core (not the radiator overflow line)  about a liter of coolant spills out and then it subsides, I top it off, turn it off and then process repeats the next time I run it.

What's going on???  Is this the normal burp, or should I pull the thermostat and test it, as well as trying to spin the water pump with the belt holding it in place to see if it's working?

If I leave the cap on everything seems fine, although it's boiling inside.

Thanks

Will Taygan
Concrete, WA