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After you get all the crud out that is possible and assuming that both returns drop into a wet return pipe before rising up into the Hartford Loop return, you could:

Add a valve to isolate the wet return from the boiler and add a full size drain valve to the lowest point on the wet return.

Then run the boiler in the "no return" mode. This is shutting the valve to prevent any return going into the boiler. Open the new drain valve to waste the return water down a drain. You will have to monitor the water level in the boiler as you do this.

The wet return at the boiler also performs as a "P" trap and is where any crud from the return lines will collect. Your "no return" mode will flush the returns.

If you go to the "Silent Steam" U-tubes by Gordo S. you will see that they often do that with a new boiler install.

After some operation, just draining the wet returns with the drain valve will remove more debris before it gets into the boiler.

Also remove the handle from the valve that isolates the returns from the boiler when finished with the no return operation.

Thanks @Steamhead much appreciated.
The steamers and one of the EKs boilers recommendations came heatinghelp.