1983 200 Merc with trim issues....

[Blueally XB21]

We have trim unit that we cannot bleed without spewing oil out the vent screw. We have done the procedure just as the manual says. It is the square motor assembly with a single cylinder system. Clearly this is some kind of mechical issue but we don't have a break down of the pump assembly so we don't feel comfortable going into check valves and the pump without diagrams. Has anyone else had this problem? We have bleed numerous pumps over the years but never encounter this issue. It's as if the resovoir is not large enough to handle the cylinder volume? Any help or suggestions is appreciated.

Thanks,
Harvey