Oddly I only got 250ml out of both of my radiators. Before I disassembled I could hear coolant moving if I shook the assembly which implied the presence of air. No leaks.
I'm flushing them using a submersible pump and a warm vinegar solution and I definitely appeared to have a blockage on one side, however it cleared fairly quickly.
The pump rate is fairly low, 5.6 liters per minute and that flow cannot be sustained through the radiator. I'm not seeing any debris. I'll have to try the 1/2 fill, connect input&output and shake method. See if anything comes out then.
I put mine on the shelf. I'll get back to it later. I clearly have a blockage in one of the halves. If I blow compressed air through the flow rate will increase to match the good half for a while but then it slows again to maybe 1/2 the rate of the good half. I ran vinegar, aluminum safe radiator cleaning solution etc through it (using the pump) but no dice.