CAV filter body /integral primer

[bluerm166]

This has been retired because of external corrosion but used for an occasional 'polishing' rig,where it filters and passes diesel with about 400 mm head of feed.
Can anyone familiar with these explain why it is barely possible to blow or suck air through the inlet (which from the embossed arrow appears either lower spigot) .It seems the priming diapragm causes this reduction in flow.
In use on the boat the filter works well enough,but is it imposing an excess load on the engine pump ?

This anomaly is very apparent if you compare an 'original' CAV body - so no primer - where,as you would expect, there is absolutely no resistance to air blown through either spigot.
Yes,I am going to fit an inline bulb with a plain body .

 

[B27]

I'd guess it's old and either full of crud or one of the valves is stuck nearly shut.

You have to rip it apart and tell us the answer!
A new one is £20 ish.

 

[Boater Sam]

The resistance to the flow required for an engine to run is minimal over the filter resistance.
However if you wanted a high flow rate through the assembly then it may become important.

 

[bluerm166]

Thanks for your comments.
Having removed the existing body ( of exactly the same design) from the boat today I find that the resistance to sucking or blowing through the inlet route ,without filter in place is excatly the same-high resistnce. Nor is the body featured above full of any crud and again ,up to the point of a recent blockage elsewhere,it worked quite happily to full revs.So it appears that this design ,with priming diapragm,has inherent resistance to flow -except when priming,and the engine lift pump overcomes the extra load.
I suppose that both bodies could have suffered the same diapragm displacement and caused exceptional resistance but that seems unlikely.
At any rate I have now fitted what is probably the original CAV manufactured body and added an inline priming bulb as others have recommended.
Others tackling a blockage or cleaning their tank might be interested to note that my blockage occurred ,not in the filter,which was clean but before that in the Wade compression shut off valve which is not full bore but reduces from 8mm to about 3 or 4mm internally .This occurred after a bracing sail hard into a 6/7 in big seas which agitated the tank to the extreme.A surprisingly small amount of crud seem here was sufficient to block the valve solid.And yes the tank has been fully cleaned.
There may be a full bore valve available for 8mm copper ,but I haven't come across one yet.