Problem with piolt on a rheem 40 natural gas hot water heater

FV system tripped... clean arrestor screen, replace TRD
bad venting
Not enough fresh air
Dirty combustion parts
Bad gas control valve
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Tripped FV or TRD, or bad gas control valve, low gas pressure, air in gas line.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
Unfortunately, Rheem does not post service manuals. Other companies post service manual, but only Rheem uses TRD FV system, while other companies use arrestor screen.
http://waterheatertimer.org/How-to-repair-Rheem-TRD.html
Add a comment and say what you discover.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

What brand, model, and approximately how old is water heater?
This is important because water heater designs are changing quickly.
Did you remove door over combustion chamber to access thermocouple?
Newer Rheem water heaters have air intake under combustion chamber that has to be blown out using compressed air.
If this is different brand, then use mirror under water heater to see air intake.
Set water heater on bricks to avoid high water. Make sure drain line out of drip pan is 3/4" and terminates in good drainage location.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Put brass garden hose cap over drain valve.
http://waterheatertimer.org/How-to-replace-water-heater-drain-valve.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If pilot lights and flame is robust and clear blue, and pilot goes out later, then some suspects can be excluded: air in gas line, low gas pressure, thermocouple is overtightened (finger-tight + 1/4 turn), thermocouple not positioned correctly in the flame, dirty pilot tube.
If flame is yellowish and small: then clean pilot tube and combustion parts.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
Remaining suspects can include: air in gas line, low gas pressure, bad gas control valve, tight house so household vent fans pull air down water heater vent that blows out pilot, hot attic with inadequate ventilation, water dripping down vent.

Open following links to read troubleshoot resources.
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Following information is for tank-type gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/pdf/Attic-Ventilation-and-Pilot-Outage-in-Gas%20Water-Heaters.pdf
http://waterheatertimer.org/images/Water-heater-in-attic-800.jpg

Probably a bad gas valve.

A leaking water heater cannot be repaired, assuming it's the tank leaking and not nearby water pipe.
http://waterheatertimer.org/Water-heater-leaking.html

First thing is to protect your property from catastrophic tank failure where tank ruptures and water runs until it is shut off:
Tanks are under pressure when they are hot.
Close cold water shut-off located above tank.
Turn control knob to 'pilot' so pilot is lit but burner will not turn on.
Open hot side on tub for few moments to relieve tank pressure.

Now your property is safe.
You need a new water heater.
In mean time, gas water heater makes 41 gallons hot water per hour.
Shower used 5-9 gallons hot water.
Bath uses 10-15 gallons hot water.
You can turn on water heater for short stretches and limp by until you get a new heater.

Steps for installing gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

you need to hold the piolit for about a minute then turn the red knob and it should lite

The thermocouple has failed. It is the temp probe that goes from the pilot lite burner area to the control valve. Go to your local plumbng repair shop and buy one. Most appliance parts stores have them also. Take the old one with you and they will hook you up with an exact replacement. They are not that hard to change. Usually a nut or 2 screws in the temp bulb area at the burner end and a nut that threads into the control valve.
Make sure before you install the new one that the area where it mates to the gas valve is clean. Should be about a 20 min job including clean up with replacement parts in hand.