1988 500 cooling - no water out of bypass

[billldz10]

The bypass valve on the left side isn't passing any water and hisses with steam after running it. I checked the lines back to the pump and they are clear.
if i hook a hose to the flush i get plenty of water out of the bypass. I even took the 90 out of the pump housing and it's clean. seems like the pump isn't feeding any water to it. The water box was split not sure if that has any effect. Any suggestion would be welcome.

Thanks

bill