Back tire/tube problems

About a month ago, I was riding in my yard (trying to get used to riding in clipless pedals) on my new road bike and had a flat. I took it to the bike shop where I bought it to get the tube changed out (I don't know how to change a flat! pitiful,huh?). Well, I have ridden it only 3 times in the past month (due to working long hours and vacation) but got it out today for a long ride. The back tire was very spongy, so I hooked up the pump. I can't get the tire hard like the front tire even though the guage reads 120.

So, what could possibly have happened? Wrong tube in the tire? It has a hole in it and is constantly losing air? I don't get it!

I am soooooooooo frustrated! I finally have some spare time to be able to ride..................and this happens!uuuuurrrrrgggg!

 

I'm assuming it's a preata valve, so in order to get air in there is a little nut on the end of it that has to be loosened(not the plastic cap that may be there). Then you depress the end fo the valve(burp it) to ensure it isn't stuck. Then you hook it up to the pump.I'm also assuming the pump is also setup for presta valves. Retighten the little nut when you are done. Tubes can loose air over time even with out a hole in them. Check Sheldon Browns glossary of bike terms at www.harriscyclery.com for the difference in presta valves and how they work. The repair section at www.parktool.com may also have some useful info. Next thing learn how to change a tube and fix a flat. Flats are part of riding bikes, and you gotta be able to deal with it or you will never be comfortable being more than a short walk from home.