fourteenbucks

I have been busy this past week and haven't been able to ride, so my bike has been sitting in the back of my car hopefully not collecting dust. The last time I rode I forgot to put the valve cap back on, and lost it when I went riding. I went today to go ride, and found that the tire with the missing valve cap was completely flat. I thought that the only way a Shrader Valve could let out air was if you depressed the button in it? Is this not the case?

Also, I did not check to see if there was a puncture to the tube yet, so that may be a possibility as well. Really, I was just wondering if it was critical that the valve cap be left on.

CastIron

Odds are the valve is fine and the lost cap has nothing to do with it.

UncleStu

The valve cap does *not* hold the air in- period- so that's not your problem. The cap keeps the valve itself, down inside the valve stem, free from dirt, mud, etc.

BarracksSi

The valve cap CAN hold air in -- if the core of the valve is a little loose and lets air leak out.

This happened with a valve on one of my car's wheels. I finally noticed it when I was in a quiet area and heard the air leaking from the valve with the cap off. I got my handy-dandy tire valve tool, tightened the core, and the leak stopped.

You can get these in nearly any automotive department or gas station:
https://www.amazon.com/TIRE-VALVE-REP...618165&sr=1-13

The end for tightening and removing Schrader cores is pointing up and to the right in that picture. Poke it in there and it's the usual lefty-loosey, righty-tightie.

StephenH

I wouldn't bet on the little plastic caps holding air in even if the valve itself was leaking. I normally toss 'em, as they're more trouble than they're worth.

mackerel

just replace the tube - they're cheap and plentiful

BarracksSi

Right, that's why I fixed the valve core instead of continuing to rely on the cap.

Velo Dog

The cap CAN hold air, but it's not supposed to. If it has to, your valve is bad or the core is loose, and you should replace the tube or tighten the core. Cars use those valves, and at least a quarter of them are missing caps. It would be pretty rare for one to fail on a bike.
You sound as if flats are a novelty to you. FWIW, they're a part of cycling. One summer here in the land of big thorns, I averaged about one every 30 miles, and I expect one a week (about 100-125 miles, this time of year). If you're unclear on the changing/patching process, it's much easier to learn in your garage than by the side of the road in the dark.
Finally, there are several sound reasons to switch to presta when you finally buy new tubes. Main one for me is that they're much easier to inflate with a frame-fit pump, because you're not fighting the spring pressure of a schrader (air pressure holds them closed). You can toss the caps on those, too. And you don't have to worry that the valve hole in your rim is too big for a presta. Just mount the tube with the valve in the center of the hole and you'll never have a problem.

Yan 

The cap has nothing to do with it. Your tire is just flat. Pump it up. If it won't hold air, patch or replace the tube.

dmac49

All tubes will loose air over a period of time. That's one of the reasons to check your tire pressure before you ride. Valve caps are not intended to hold in air, but merely to protect from getting debris in the stem. I have no doubt that caps can slow the lose of air if they are on tight etc. High pressure tires tend to loose pressure faster than low pressure ones. As stated before...check your tires and pump them up before you ride. You can also change the valve in the stem for very cheap. Put some spit on the top of the stem after you pump it up. If it bubbles replace the valve.

BarracksSi

And, IMO, everyone who has Schrader valves, whether on a car or bike, and isn't afraid of dead-simple mechanical effort should have one of those tools I linked to earlier. It's got a driver for removing and installing valve cores, a reamer for cleaning out the stem, and taps for both inner and outer threads. Any problems that need more than that also mean getting a new tube anyway.

Retro Grouch 

Zowie!

Just how much trouble do you have with your Schrader valves? Reaming out the stem? Retapping threads? If you're doing all that and are still having trouble, maybe this is one of those "If it ain't broke don't fix it" things.

I'm 66, and I've been fooling with Schrader valves on my bikes, cars and air conditioners for that matter, all my life. I've never found the need to do any of those things.

BarracksSi

You'd think that you would have learned by now that if you need to fix something but can't, you're screwed.

I-Like-To-Bike

How much trouble could those caps cause? You sure it isn't the extra grams causing you the trouble?