CAT equipment like the 120M road grader, 924H wheel loader, D7R II dozer and 621G scraper all use the same ether start aid valve that comes with NSN 4820-01-551-9292.
Inside the valve is a filter that gets clogged with debris. Sometimes it gets damaged or just plain disappears. If you look for the filter in the TMs, it’s not there.
Start aid valve filter
All is not lost though. You can now order just the filter with NSN 2910-01-658-4647. The filter costs just over five bucks. That’s a big savings when you consider that a new valve is $157!