While at a NAPA store, I bought some valve caps that indicate tire pressure by showing green, yellow or red behind a clear cover. It seemed like a bargain Tire Pressure Monitor system, until I noticed a flat front tire while riding one day. I found that the clear cover of the valve cap had broken off, and the air had escaped. I don't know when or how, but, after I ran my portable pump to get enough pressure, I found a plain old metal valve cap in my parts bag, and limped to a service station where I re-inflated my tire to full pressure. It has stayed inflated ever since. There was no harm done this time, but I am suspicious about these valve caps. Has anyone had a similar experience?