I have a Whiperflo 3hp pool pump. The water

if the pump is above water level there is no need for valves. If the pump is below water level, partially drain pool until water level is below pump, then install unions and shut off valves when you remove the pump.

water only goes through sight glass when backwashing check your chemicals make sure ph and alkalinity are within spec also check chlorine total and available sounds like you need to super shock pool

AIR LEAK IN SUCTION LINE. PUT A GARDEN HOSE WITH A RAG AROUND IT TO BACKFEED FROM INTAKE IN POOL. WITH PUMP "OFF" TO LOCATE BREECH IN LINE.

Total alkalinity can be lowered by adding muriatic acid or sodium bisulfate. Acid may be added in doses of up to 1 quart per 10,000 gallons. Total alkalinity tests and further required additions of acid can be made every 2 hours.

I happen to have a copy of the users manual for this pool filter. These are the instructions for operating your filter.

1. Be sure correct amount of filter sand media is in tank and
that all connections have been made and are secure.
2. Depress Vari-Flo control valve handle and rotate to
BACKWASH* position. (To prevent damage to control valve
seal, always depress handle before turning.)
3. Prime and start pump according to pump instructions (be
sure all suction and return lines are open), allowing the filter
tank to fill with water.
WARNING: ALL SUCTION AND
DISCHARGE VALVES MUST BE OPEN WHEN
STARTING THE SYSTEM. FAILURE TO DO
SO COULD CAUSE SEVERE PERSONAL
INJURY.
Once water flow is steady out the waste line, run the pump for
at least 2 minutes. The initial back-washing of the filter is
recommended to remove any impurities or fine sand particles
in the sand media.
4. Turn pump off and set valve to RINSE position. Start pump
and operate until water in sight glass is clear—about 1/2 to 1
minute. Turn pump off, set valve to FILTER position and
restart pump. Your filter is now operating in the normal filter
mode, filtering particles from the pool water.
5. Adjust pool suction and return valves to achieve desired
flow. Check system and filter for water leaks and tighten
connections, bolts, nuts, as required.
6. Note the initial pressure gauge reading when the filter is
clean. (It will vary from pool to pool depending upon the
pump and general piping system). As the filter removes dirt
and impurities from the pool water, the accumulation in the
filter will cause the pressure to rise and flow to diminish.
When the pressure gauge reading is 8-10 PSI (0.55-0.69
BAR) higher than the initial "clean" pressure you noted, it is
time to backwash (clean) the filter (see BACKWASH under
Filter Control Valve Functions.)

You have a clogged impeller

A better way to clean the filter tubes in the EC40 is to remove the filter tube assembly and soak it in a 5 gallon paint pail that is filled with a solution of water and a 20% muriatic acid, available at hardware and home center stores. Be very careful handling the muriatic acid. Don't splash it around.

Mix it in a 1-to-1 ratio with water and soak the assembly for at least 8 hours. Keep the soaking pail out of the reach of kids and animals.

Remove the assembly, properly dispose of the acid mix and refill the pail with clean water. Soak again for 3 to 4 hours, then reassemble your filter.

Some pool guys say it isn't necessary to soak the tubes in clean water because the muriatic acid will not harm the pool water and will reduce the pH if it's too high. I've always soaked them to be safe.

This is a link to the Hayward manual that describes the process:
www.haywardnet.com/products/Manuals/pdfs/manual9.pdf

The pressure you are reading is the pressure in the filter tank as water is pumped though it. When the filter is freshly charged, 9 PSI is a good pressure.

As the filter removes particles from the water the filter gets "dirty" -- it takes more pressure to push the water through it thus raising the PSI.

I think you are viewing this in reverse. Low filter pressure is "good" (as long as it stays above 8 PSI) -- as the filter cleans the water the PSI will increase -- when it gets to 15-20 PSI it needs to be back-washed and recharged.

The pressure drops when the pool cleaner is connected because pool cleaners always pull in air (in varying amounts) because of all the hose connections. This causes your filter/tank pressure to drop.

fix any air leaks on suction side of pump. Check valve positions.
Clean filters...may need to replace cartridges in cartridge filter, or take out D.E. grid assembly and clean it on a D.E. filter, or change sand in a sand filter...

not a easy task, the bearing are easy to damage . try looking for your local pool pump motor repair shop (your zip code) cost should be around 85.00 with a new shaft seal (a must !)