Could be limescale build up due to our hard water.
If you don't run an acid rinse, go to Hugh Crane and get some "Phos" descaler, mix it about 50 to 1 and run it through with the heater on for about half an hour. You can also run it through your tools into a bucket.
When you've done run plenty of fresh water through everything.
If you do run an acid rinse, your pump may need re-building or changing.