We have Racor's vacuum gauges on our Racor 500s that are ahead of our Ford Lehman 120s and, like everyone else I've talked to who has a similar setup, at a cruise rpm of 1600-1700 rpm the gauges read exactly zero. The engines don't pull enough fuel at typical cruise rpm to produce any vacuum reading at all with clean filters. But this is with the FL120. I don't know anyone with gauge-equipped FL135s so cannot tell you what a typical reading should be. You might call American Diesel for their advice. I do know that AD will tell you that you shouldn't be using Racor 500s with any Ford Lehman engine as they feel the engines don't pull enough fuel (particularly the 120) to make the fuel swirl in the "turbine" section of the filter. They say you should use the Racor 1000 because since there will be no swirling action to help separate contaminants the only filtering you'll get is from the element itself and the 1000 has a larger one. Racor emphatically denies this and told me that just about any fuel flow rate will set up the swirling motion in a 500. And our diesel shop never installs Model 1000s on FL120s unless specifically instructed to because they say it's an unnecessary expense and that the 500 does a great job. I don't know if they recommend the 1000 for the FL135. While I wouldn't rule out a clogging filter as the cause of your problem (and it's easy enough to change the element and find out) the symptoms you describe sound more like air getting into the fuel. Short of throwing a rod, nothing will stop a diesel faster than air in the fuel. Even a tiny, intermittent air leak can cause an engine to stumble, drop it's rpm randomly, and die at lower rpms. ____________________ C. Marin Faure GB36-403 "La Perouse" Bellingham, Washington