Pop off valve dripping Rheem 50 gal electric valve at bottom

If you are talking the P&T valve, then yes...about 10 dollars @ the hardware store. You may want to think about an extrol expansion tank if your city water give you pressure problems.

Copy following link for Rheem marathon parts and resources:
http://waterheatertimer.org/Review-Rheem-Marathon-water-heater.html

Add a comment March 2013 and describe problem in detail.

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

Unless you are getting more than 150 PSI, you should just replace the valve. It has dirt in it.

If the drip is coming out of drain valve: Put a hose cap over end of drain valve.
If water is seeping out of cracked drain vale, then replace it.
If the leak is seeping around previously sealed threads, then tank may be rusted beyond repair.
http://waterheatertimer.org/How-to-replace-water-heater-drain-valve.html
http://waterheatertimer.org/Leaking-water-heater.html

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

There should be a valve on the water supply line just above the water heater. There may just be a drain spicket located at the bottom of the water heater that is leaking. You may just be able to put a cap on it or try to turn it tighter to stop leaking.

Water heater should be set in a drip pan.
Otherwise, put 1 gallon bucket under TP release pipe to catch water.
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#drip-pan

When you say relief valve, I assum you are talking about brass valve located on top of tank or near top of tank. This is TP valve and a pipe connects to this valve and runs down to within 6" of bottom of tank. TP valve has a small lever on it.
If you are talking about tank drain valve located at bottom of tank, then add a comment.
Following information is for TP valve.

Dripping TP valve can be caused by 3 different thing:

1) Obstruction inside TP valve. Open TP valve for a moment to flush away particles and slowly lower lever back down. See if water stops dripping, if not, then replace TP valve

2) Defective TP valve: replace TP valve. Turn off electricity to water heater. Let heater cool for several hours so you do not get scalded. Close shut off valve located above tank on cold side. Open tub spout to relieve pressure, open tank drain or open TP valve and empty water below TP valve level. If TP is located on top of tank, not much water has to be drained. Tank is under press when hot, so be careful. Put teflon tape on new valve and install with big pipe wrench by turning clockwise until TP valve is very tight.

3) Overheating tank. TP valve is temperature-pressure valve. If tank water is over 150 degrees, the TP valve starts drizzling hot water to relieve tank pressure.
Lower tank pressure two different ways:
a) turn down thermostat
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
b) install expansion tank on incoming cold water line
http://waterheatertimer.org/Water-at-bottom-of-water-heater.html#expansion-tank

The plastic drain valve is a possibility. Also the Temperature/pressure relief valve is typically piped close to the floor if there is a drain near by. People tend to ignore water heaters. The TPR valve really should be excersized annually. If it won't re-seat, then it should be replaced. Second the tank should be partially drained annually to drain any solids that have accumulated in the bottom over time. If you don't do this frequently the drain valve can become almost impossible to operate.

If you mean the safety valve, replace it. They're only a few dollars.

Sounds like pressure building up in your line to much. Did you make sure to get all of the air out of the system by opening up your relief valve?

If your water tank is leaking, is it dripping from above or actually leaking thru the bottom?. Can you find the source? Feel around and try to find wet spots above and around it. If it's not leaking from the water pipes or T&P valve, it's your tank leaking. You'd be wise to have it checked out in the morning, before it gets worse. Hopefully your catch pan has a pipe attached to divert water to the outside or to a sump pump or floor drain. Water heaters do sweat, but you can tell the difference if it continues to leak for several hours.