I've had a few experiences with 850 EGR valves and although the OEM service manual will get one to properly diagnose problems with the system such as testing vacuum devices, the most common problem I've seen with high mileage 850s is that the hole in the intake manifold simply clogs with an oil/carbon type debris and it becomes very solid.
To access that, the pipe must be untreaded from the valve, the throttle base removed and then remove the EGR valve completely (SAVE the gasket or replace).
With the EGR valve and throttle base removed, if that hole is clogged as I'm suspecting, you'll see it and it'll be obvious. Use a drill bit, screwdriver or sharpe object to open that hole up and your problem should be cured.