LeAking pressure valve on water filter housing

Was it leaking before changing the filter? It could be that the water valve in back is leaking a bit. Sometimes the valve gets a very small bit of grit in it and doesn't completely shut off. The way to tell is to remove the hose from the valve and see if water is dripping from it at the valve. If it isn't then maybe the water reservoir is partially frozen and causing a pressure build up in the reservoir. If this is the case the dripping should stop after a couple minutes. I would look at the valve.

Check all connections for air leak including the water drain valve with soapy water. If no air leaking from connections, spray soapy water on head area where head attaches to the cylinder checking for gasket leaks. If no leaks are found, most likely problem is in the head /valves /internal gasket leak. If head gasket or valves are leaking, you will notice that air seems to blow out of the intake filter (normally there is good suction at the intake). Good luck

Question is unclear.
What is a vacuum valve?
You posted under electric water heater.
Water heaters are associated with TP or 'safety overflow' valve, water shut-off valve, tank drain valve, pressure-regulating valve, mixing valve and possibly a few other plumbing valves.
Vacuum valve implies that a compressor is creating pressure on one line while creating vacuum on another line. I don't know of a water heater that uses air compressor.
Open following link to help troubleshoot leaking water heater:
http://waterheatertimer.org/Leaking-water-heater.html

Air leaking back from tank check valve or leaking reed valves can make a squeaking or vibrating noise. With pressure in tank remove fitting at the tank check valve and test for leak with soapy water. If leaking, replace with generic valve. If no leak, reinstall fitting and let all the air out of the tank. With empty tank and air filter removed, start compressor and feel strength of suction at intake. When you hear the squeak retest strength of suction at intake and compare. If reed valves are leaking, you will feel less suction as tank pressure increases, and in severe leaking valves, you will actually feel air blowing out of intake. Repair/replace reeds and /or leaking gaskets in head of compressor. Good luck

It sounds like two problems.
Problem 1) Clog inside faucet or clog inside pipes at faucet
Remove supply tubes leading to faucet and see if problem is inside pipes or inside faucet.
My bet is clog inside faucet.

If you have galvanized pipe that meets copper pipe, this can be trouble spot.
Article on clogged water pipes at Inspectapedia:
http://www.inspectapedia.com/water/Water_Pipe_Clog_Repair_Guide.htm

Article on clogged pipes:
http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html

Problem 2) Water heater leak ... you say tank valve is leaking? Is that cold-water shut-off valve? Or tank drain valve? Or TP valve?

If cold water shut-off is leaking, then replace shut off.

If tank drain valve is leaking, then check if plastic valve is cracked. If drain valve is cracked, then replace with a brass valve from Home Depot. If plastic valve breaks off, then use hammer and screwdriver to gently chip out the broken plastic valve. New valve threads need teflon tape to seal pipe. If valve is just dripping, then put garden-hose cap over end of valve and screw down tight.

If TP valve is leaking, then replace with same temperature and pressure rating. New TP valve threads need teflon tape to seal pipe.

Problem 3) If pressure is low all over house:
Buy pressure gauge at hardware store that screws onto hose connection.

Test pressure on outdoor spigot.
Open faucet and check if pressure drops. This says if problem is inside pipes that enter house.

Test pressure on water heater drain valve.
This says if pressure problem is before -or- after water heater.

If problem is before the water heater, then shut-off valve located on cold water line is suspect.
If problem is after water heater, it could be in hot water outflow pipe on top of water heater, or a nearby elbow.
Remove hot water line leading from water heater and check pressure. This says if problem is where hot water leaves tank.
Read articles above about clogs inside water lines.

Turn the water supply to filter system off, open the faucet to relieve the pressure.
It depends are where it is leaking form. If you just changed the filter it is either not on all way, the o-ring is pinched or missing, or the inlet line, outlet line (or both) were worked loose and are leaking.
Check the feed valve to the filter also. If it is a saddle/ piercing valve it may be leaking. After opening and closing these valves several times the "packing nut" may work loose and the valve leaks. Tighten this very slightly until leak stops. Here is a diagram of a saddle/piercing valve.



The packing nut is right below the T handle as in this picture. There is seal under the packing nut and this is where it can leak from when the nut works loose. AGAIN tighten this very slightly to stop the leak.
RJ

what is the water pressure on the cold side? You may need to install a pressure reducing valve at the water heater inlet to reduce inlet pressure and keep pop off valve from opening when not needed.

There's probably some particles that got trapped in the filter purge valve ( the red button) preventing it from closing all the way.

You may be able to clean the valve while the filter top is still in place. Try using a small brush. A toothbrush may work if you cut off the handle. Hold the button down while you clean the inside of the valve. Then take a
spray bottle and put the sprayer tip on stream. Flush the valve from the inside.
If that did not do it, next step is to remove the top from the water lines, and flush the valve out.

The red button is a pressure relieve valve. If the system is leaking from the valve, try pushing the valve up from the underside of the cap where the valve is located. If this does not work the housing needs to be replaced. You do not have to replace this with a name brand like GE. All you need to do is make sure the housing that you are purchasing is the same size.

Example of Replacement Filter Housing: http://www.freshwatersystems.com/c-107-filter-housings.aspx

FreshWaterSystems.com: The Water Super Store: 1-877-335-3339