There has been lots of info on fuel filters and their problems, this info was lifted from the SSCA board,might be of use.
Re: Issue With Dual Racor Fuel Filter System
» Tue Feb 09, 2016 5:04 pm
Air accumulation in a Racor system, especially if the motive force is VACUUM, is due to:
99% probability is an air LEAK ....
1. incorrectly installed (gasket, o-ring) on the Racor.
2. The gasketing/o-ring was NOT lubricated with either fluid compatible 'grease' or diesel fuel during installation of new filter cartridges, causes a high probability that the gaskets/o-rings to 'pinch' and thus deform during assembly of the filter housing.
2a. If your fuel dealer is delivering so-called 'bio-fuels' including 'mixtures' of so-called biofuels, especially bio-fuels made from 're-claimed' cooking oils ..... the typical BUNA/Neoprene rubbers found on Racors, etc.and other compoents of the fuel delivery system are NOT COMPATIBLE with the organic acids found in bio-fuels - will cause softening, swelling and change of geometry of these seals, gaskets, diaphragms. If so, then change all the gasketing, etc. to VITON rubber components - expensive.
3. The usage of 'compression fittings' - are notorious 'leakers', especially in vacuum motive systems. Vacuum motive fuel systems should have a minimum of double-flare (ring seal) connectors.
Worse, compression fittings will 'self relieve' over time - the brass teeny 'ferrule' within a compression fitting ultimately depresses/deforms the copper tube resulting in a leak between the ferrule and the tube; thus, almost absolutely requiring that the tube be trimmed back and the 'ferrule' - only to be re-installed over 'fresh' copper .....EVERY damn time you loosen or disassemble a compression fitting.
4. Slip-on joints between rubber hose and tubing - notorious 'air leakers' in vacuum systems. Worse, the 'dead space' between the hosing and the tube will invariably become a breeding space for fungus and bacteria which will continually 're-inocculate' the fuel system, and whether a 'biocide' is regularly used or not.
the fuel distribution system from the tank to the lift pump with 20-25 psi air, apply a soap/detergent mixture to all joints and along any 'old' copper tube. Leakage are will be visualized by 'bubbles' in the soap solution. Replace all 'connections' with double flared connectors (as a minimum).
reasons (1%) for air accumulation in a Racor, etc.
1. Filter system 'geometry' - Such a filter for diesel fuel should be at the very LOWEST altitude in the system !!!! (bilge or other low altitude space).
Reason: entrained air in diesel fuel, either from 'fast filling' the tank or insufficient baffles inside the tank causing undue 'impact' with the tank walls will cause a biphasic mixture of fuel and air as an 'emulsion'.
It takes a VERY long time for such an emulsion mixture to release and separate the air emulsion. If fuel with an air emulsion is drawn into the fuel delivery system these small microscopic and macroscopic air bubbles will continually 'coalesce' into larger and larger bubbles (especially inside the filter media where 'coalescing' is most active) ..... until the 'buoyancy' of the now larger air bubbles drives them to the 'highest altitude of the entire system' .... there to remain as a potential large 'slug' of air.
If the fuel filters are located at the very bottom 'altitude'
of the fuel delivery system any air that 'drops out' will go in two directions upon shut down of the system: ... 1. back to the tank, or 2. to the 'guard' filter mounted on the engine (hence into the engine) ... most modern diesel engines will tolerate a 'small' amount of air, especially Yanmars.
Note: With the filters at the VERY bottom of the system architecture, any water that is mixed into the fuel will, from either direction (engine or top of the tanks dip tube), will drain forward or backward into the 'sump' of the filter housing. With the filter at the top of the system geometry, water 'will' collect IN the fuel lines (rather than the filter 'sump'), until ultimately the flow requirements of fuel allow/cause a slug of water to flow through to the engine, etc. ... and usually shatter a hot injector tip(s).
As regards drain-back check valves ..... not an issue as once the system is fully filled (typically inside 1/4' ID tube/hose) the surface tension/viscosity will cause the oil stay there due to 'capillarity' .... unless there is a leak - air leak; plus if the bulk of the system architecture is below the top
of the tanks dip tube ... well, fluids don't 'drain uphill' and even if a leak, only the volume in the dip tube will drain back if 'capillarity' is insufficient.
For 35+ years I was deeply involved in advanced filtration engineering, including indirect employment with the parent organization of "Racor".
99+% probability you have a system that is leaking air !!!!!!!!!