Two things I did
AlexPeterson said:
What purpose does a filter in this line serve?
I had massive problems with my RV-4 and a old fashion mechanical MP gauge. The gauge got contaminated with an oily gas mixture, which migrated down the hose. My solution was two fold, A restrictor fitting and a service loop so to speak.
Originally, I had an AN fitting in #3, hose and fitting in firewall and a hose to gauge. The hose pretty much went straight from Cyl to firewall. All fittings where standard AN fittings.
First, I modified the AN fitting with restrictor plugs on both the cylinder and firewall AN fittings. I made them with a rivet and a small 1/32"-1/16" hole drilled in it. The reduced orifice had no affect on the gauge.
The 2nd part was to extend the hose and make a service loop. After leaving the #3 cylinder intake port, I used a longer flex hose that went down towards the bottom of the firewall to a Tee fitting held in a bracket. One leg of the Tee was capped, the other leg went up to the original firewall to cockpit fitting, using a ridged aluminum tube. I would open the cap on the Tee and drain a few drops of mosture/gas/oil mixture every once and a while. problem solved.
Doing both restrictor fittings and hose re-route at the same time its hard to say what did the trick. However I got very little liquid in the hose/service trap. Doing it over I'd just go with a longer hose with a good elevation drop/loop or dip. Make sure the MP fitting is high in the firewall. Probably a good idea to clean out the lines on occasion, which should take a few minutes.
I think a restrictor fitting on the Cylinder is the first line of defense. Clearly its in the intake port area where there is fuel/air mixture. It only makes sense some will find its way into the hose. The restrictor and the LOW POINT in hose method works well, with out a filter.
This fuel in the MP gauge is not an uncommon problem, I had a factory plane blow gas into a MP gauge.
Filters? I think its not needed and might cause problems. Looks like the one above is for pitot/static. Not against it or saying it would not work, just I like to keep it simple. The restrictor and loop/dip is good enough for me. However YOU have should clean the hose out on occasion, just like a filter, if that is the way you want to go.