The relief did it's job. The leak under the sink was from pressure most likely before the relief opened. I would need more information. Are you on a private well or city water.

Call a plumber if you do not want to try and do the work yourself.

Lowe's sells supplies, and in some places subs out repair jobs.
If you go to a 'Do It Yourself' place they sort of assume you are interested in doing it yourself.

do you have a pressure reducing valve on your system?
something causeed the system to hit 185psi
thats what you need to find,the cause of that
every thing else was just a reaction
change the pop off for the water heater, not the whole heater...inless the tank is busted,,but 7 years old is not to old...clean out the calicum while you have it off line

Hey thanks for the responses!
We have city water but we live at the end of the line down hill--they upped the pressure about 2 yrs ago because of low pressure complaints and bacteria in the lines etc.

The people at Lowe's said they have seen a lot of business and thought it was because of the constrction going on nearby--we have a small hotel being built and townhouses etc.

No pressure reducer that we know of and we are the original owners & have lived here 18 yrs. This is our 2nd water heater--things corrode here pretty quickly.

Just replacing the T&P valve on the water heater will not, by itself, help you when your water pressure is above the preset pressure limit (and 185 PSI is WAY above). You'll need to install a pressure regulator on your incoming water line, as well as an expansion tank, in order to get your water pressure within proper operating limits, and to allow for some normal thermal expansion of the water in your household plumbing.
Your T&P valve on the water heater did its job and may not need replacing as it should close when the water pressure is below 135PSI.

I don't know if it's corrosion of your water heater (unless the sacrificial anode has failed or is missing) but your water pressure is a likely culprit for the premature failure.

"Normal" water pressure is around 60psi, 70-odd is good, but your water heater will be rated for a MAX of 100psi. The T&P valve on the tank did its job and should have been leaking from day 1 if that pressure has been like that - it's almost double the max.

You should also get into the habit of lifting that little lever on the relief valve monthly or so, and letting some water run out. If it doesn't stop straight away, leave it a while, if it keeps dripping open it full up and let it run longer. This flushes out any material stuck in there and blocking it. It's very important to do this flushing, if the valve sticks, and you have high water pressure like that, (or even if you don't) you can get a dangerous buildup of pressure that can make it explode - you can even find video footage of it on youtube and mythbusters.

What you should add (they are a good idea even if your water pressure is ok) is a pressure reducing valve like the link below - it will allow you to reduce your pressure to something manageable, the gauge your hubby bought will be handy.

With a pressure reducing valve, you will also need an potable water (not the non-potable which are for boilers and underfloor heating) expansion tank for the hot water, it will also work as a shock absorber for the whole system like the hammer eliminators you get for the washing machine.

Expansion tanks: http://www.watts.com/pro/_productsFull.asp?catId=64&parCat=325&pid=852&ref=2

With your pressure under control and an expansion tank you will get much longer service from your hw heater and other fixtures. The reason why your stuff is leaking may be a combination of the excessive pressure, your hw tank pressure relief valve may have been stuck, but the above is the answer. Let us know how you make out.

You might enquire with the city what's up with the pressure, but it may be 'normal' to help cope with your neighbourhood demand.

Here is a link that might be useful: Watts pressure reducing valve

Thanks everyone for all the help--we had the plumber out and we were able to explain to him what exactly was happening & to understand his advice. "pjb999" thanks for your very good information. We are following your advice to the letter. The relief valve on the water heater blew off during the event and we decided to get a new water heater. It was raised up off the ground about 6 inches(I hate to say that the former one was at gound level sitting on a tiny concrete pad, mud all around). There is an expansion tank--it is now the code--something we had never heard of. The pressure reducing valve will be installed tomorrow. We now know that we should have been checking under the house monthly. The plumber said that the pressure problems were common in our town because of all the new neighborhoods below us--they had to crank up the pressure to get water to those houses.
This has been a real wake up call for us--we've been warning all our neighbors to check their water heaters!
Thanks again for all the help.

Glad you're getting it sorted out, and nice of you to warn the neighbours - that pressure could be downright dangerous.

I think replacing the hw tank is a good move, the old one would literally be on its last legs, working at that pressure. The fact it lasted this long is actually a good recommendation for that brand!

Funnily enough, Today I saw the rerun of the 'water heater rocket' on Mythbusters. Quite spectacular.

Hey--one more thing and a big thank you!
We're done here--today they came out and put the pressure reducing valve on and we can tell a major difference in how the water comes out of the spigot. Instead of being doused with splashing water, we get a nice well behaved stream of water--who knew! We love the difference. Now we can relax until the next catastrophe.
I've seen that episode of Myth Busters and I'm happy that that didn't happen under my house.

water pressure commenly rise at nite when the system is not being taxed..but not to 185psi..it will fluctuate to 100 psi
from 50 or 60.. commen thing