ASI and pitot checkout

I have a 4500 installed and am the process of checking system operation prior to closing up the forward sheet metal on an RV-8. I have set up a water manometer to check pitot line integrity and ASI calibration. One side is connected to the pitot line in the wing (not the pitot tube) the other side to a syringe that is used to create the test pressure. The water column rises when pressurized and a scale calibrated in inches allows me to read the pressure on the pitot side in inches of water.
When I pressurize the manometer and read inches of water I get the correct corresponding ASI value. 26.63 in H2O produces 200 Knots. I also have a Dynon D6 installed as a backup and it agrees within 2 knots of the 4500.
With the system stable, that is the water column remains static the ASI begins to indicate a decline in indicated airspeed (as does the Dynon). The rate is about 1.5 knots/minute. After 10 minutes the water column remains unchanged but the airspeed indication has gone from 200 knots to 175 knots. All this is setup in a temperature stable environment with ships power be supplied by a DC supply set at 14 volts. The static system is installed and while I haven't checked it for leak down it makes no difference whether it is connected to the static port on the 4500 or Dynon.
All initial ASI indications correspond to appropriate water column heights from 40 knots to 210 knots. Since pitot is dynamic pressure in flight the ASI would initially read correctly but once in a cruise condition the ASI would show a declining airspeed. Any insights would be most appreciated!