Pilot won't stay lit, replaced thermalcouple but

Replace the thermocouple.

Reliance water heater?
Thermocouple is good if the pilot stays lit continuously.

501 is older model. Unknown if this unit has FVIR protection and sealed combustion chamber or open air intake on front of unit.
http://waterheatertimer.org/pdf/Reliance-2006-Service-Handbook.pdf

With FVIR protection:
Get more fresh air to the unit.
Clean the flame arrestor screen.
Test and replace the thermocouple:
http://waterheatertimer.org/How-to-test-water-heater-thermocouple.html
Inspect and clean burner and combustion parts
Test gas pressure, bleed air out of gas line.
Replace gas control valve.
Test draft air by lighting match and see if smoke is pulled into the vent pipe at draft hood on top of tank.

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

Gas water heaters don't have elements, so I am confused about what type water heater you have..
Look on the side of tank for the label.
Find the brand, and type. Does it show BTUs? Or does it show wattage?
Then re-post question with this information, for best possible answer.
Also post question on Tank forum.
http://www.thetankatwaterheaterrescue.com/forums/forum3/

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

Possible defective gas valve.

Make sure thermocouple tip is seated inside the burner flame.
Make sure thermocouple is not overtightened. Finger-tight + 1/4 turn.
If the pilot lights, then ECO energy cut off is not tripped, but gas control valve might be bad.
Check color of pilot flame: it should be strong and clear blue.
If pilot flame is weak, or color of flame is yellow-orange: Clean pilot orifice and burner and combustion parts on yearly basis.
Clean air intake screen if applicable.
Increase air supply to water heater. Open a window and see what happens.
Check for adequate draft going up chimney using match under vent hood to see if smoke drafts immediately upwards.
Bleed air out of gas line.
Check gas pressure.

Open following links for troubleshoot resources:
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/pdf/Pilot-outage-troubleshooting.pdf
http://waterheatertimer.org/pdf/Vent-spillage-test-atmospheric-gas-water-heater.pdf
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

1) Get more air to the water heater and see if problem continues.
2) Read the following pdf-s for things that might apply to your situation.
http://waterheatertimer.org/images/Negative-pressure.jpg
http://waterheatertimer.org/pdf/Attic-Ventilation-and-Pilot-Outage-in-Gas%20Water-Heaters.pdf
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

3) Read pdfs above first, and then Add a comment with missing information below, for possible do-it-yourself help.
Does this water heater have a blower?
Is gas valve electronic? Or does gas valve have a dial?
If gas valve is electronic, have you checked outlet with circuit analyzer?
Did pilot light work correctly for a while? If so how long?
Where is water heater located? Hot attic? Closet? Garage? Is area dirty and dusty? Has air intake been cleaned?
What color is pilot flame? Clear blue or yellow-or-orange?
Have you inspected inside of combustion chamber for dripping water?

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

Most gas water heater problems can be solved by having service technician perform yearly maintenance. Or by doing maintenance yourself.
This maintenance includes cleaning burner parts and combustion chamber, checking gas pressure and supply, bleeding air out of gas line, checking vent draft, making sure air supply is adequate and not exposed to chemicals, cleaning air intake. Also adjusting flame sensor so it is positioned correctly. If water heater is connected to electric circuit, the outlet must have correct polarity.

You can get gas to appliance indicates the gas control valve and water heater are not locked out. But the gas control valve thermostat can still be bad.

Open following link for illustrations, typical service manuals, and maintenance information, and images of gas control valve thermostats.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot