Watts 9DM3 back flow preventer/new model number LFD9

I was wondering how easy it is to clean the Watts 9DM3 back flow preventer?

I have softened well water, my Watts back flow preventer is mounted on a long nipple 1/2" black iron pipe after the ball valve and before the Watts fast fill valve on the pump module.

I wanted to fill the steel compression tank with water as it appears low on water after I installed the new Bell & Gossett NRF-25 circulator.

I opened the ATF 12 to vent the excess air out of the steel compression tank then I opened the ball valve to add water back and the fast fill valve but nothing happened other than air coming out of the steel compression tank.

I have not had to add water back to my coal stokers system for at least 5 years before this.

There was no leakage coming from the back flow preventer nor was there any pressure build up on the boiler pressure gauge.

I can buy a new back flow preventer and just swap them out I guess using the old unions if the long dimensions for the valve length are the same.

I found a great deal of dirt and rust in the isolation valves when I removed the old circulator and I cleaned that out and I also sanded the flanges smooth as they were very rough with 10 years of seasonal use for heating only.

Investing in a dirt/magnet separator should be on the list when the plumber comes on Wednesday; The only issue is where to locate it as the boiler will probably have to be drained to sweat in a dirt/magnet separator on the return loop to the sump.

That would allow for the installation of a Drain O-Tank valve and reducing bushing at the front of the steel compression tank after the plug is removed.

I should probably buy 2 cans of Fernox to have on hand before the work starts then I can charge the boiler through the boiler drain on the pump module Tee that is used to vent air from the system.

Thanks much,

Leon