Haskel now offers their EXT-420 plumbed so that you can run it in either single or double stage, depending on where volume or pressure are required. They did so using check valves and a single two way valve.
For ya’ll who have the 59025-3 or standard EXT-420, here is how WolfWurx made them selectable using two three way valves. Two valves to switch instead of one, but ostensibly avoids the pressure losses that accompany spring checks.
![](https://i0.wp.com/graywolfslair.com/wp-content/uploads/2023/04/Schematic-jpg.webp?resize=640%2C382&ssl=1)
Plumbing schematic
![](https://i0.wp.com/web.archive.org/web/20211026135042im_/https://graywolfslair.com/images/Converted.jpg?w=1170&ssl=1)
Converted Haskel EXT-420