I've used these units for about 10 years, been happy with them and water tested after the unit has always been good. Filters and a push button quick connect which makes changing them a breeze.
https://www.premierh2o.com/531411-ro-pure-guide
I have a friend who had a very similar looking saw (I forget the model #) it didn't have a valve for down feed pressure. It had some spring assist that didnt work well, was great for ripping teeth of the band if you didn't baby sit it.
Check this site out, not saying it matches (the CAT stuff) what you have now for a replacement but worth to give a look. https://m.vevor.com/s/excavator-seat