I had the same problem....it's cavitating in the timing cover.

You'll have to take the plate off that covers the timing cover and get a piece welded in, and prevents the water to circulate in the cover, that forces the water through the heads.

I think the piece that got welded i was about 1/8" x 1".

Check it and see, that was the reply I got to the very same question.

good luck