Mud build-up in boiler

I was just wondering if anyone could help me out with a steam system that every year fills up with mud.  Granted, we don't have a lot of these systems out there but this one customer has more than his share of service calls because of it.  His system is a gas fired steam boiler that has two zones of hot water baseboard radiation.  The mud gets into the impellers on the circ pumps and destroys them.  Last year we put a Taco 110 on to try to see if that would help, but we were back today replacing a leaking bearing assembly.  We flushed it again today.  I did tell him if I could come up with any other information I would let him know.  This was the only place I thought of. 

Thanks much.