So, I thought it would be a good idea to get those little doo-dads that screw onto your valve stem cap and show you that your tire pressure is okay based on a colored flag system.  I just look to see that all tires are "in the green" and we're good to go.  We were in a hurry the other day, and hooked up and loaded th horses. Lucky enough for me, I looked at the tire signals, and one was in the red.  Sure enough the tire was nearly flat. I filled it back up, and when I went to put the tire signal back on, it didn't stop hissing.  The signal cap was cracked or faulty in some way and drained all the air out of my tire.  Thank God my new tire wasn't defective. These tire signals are made of plastic, and can evidently fail suddenly and let all the air out of your tire.  Do not buy these. It's not worth the convenience.  A good tire pressure guage and normal valve stem caps are much more reliable.  However, I wouldn't have noticed that the tire was low if I didn't see the red in the tire signal.  Valuable lesson learned.  Don't be in a hurry, you won't save any time. 

Amanda

This is a much better system...

http://www.mrtrailer.com/pressurepro.htm

Checking your tires before each trip is the safest way to lessen the chances of a blowout. The system Paul offers links to, allows constant monitoring. This would be the best system of all.

Many people don't realize, that many tires can loose half of their air pressure, and not look like they are underinflated. This happens more so on radial tires than biased ply tires. You can never assume by a visual inspection or a push on the sidewall, that a tire, is in fact, properly inflated. Purchase a good gauge and use it often.

Gard