Leaking coolant - bleeder valve housing 911/996 3.6 2003

[Hilux2400]

I have a small coolant leak from the gasket interface on the bleeder valve housing which sits on top of the expansion tank. I have given some of the bolts a half turn, but there is one that is stuck. I am afraid to turn it too far in case it shears. I have given this offending bolt a slight tweak in both directions, but no luck.

Does anyone have any ideas?

To replace the bleeder valve I need to move the header tank together with the SAI pump and I am hoping that I would be able to just tighten things up.

Thanks for reading.

H