I've had a very slow leak in my front tire for a while now. By slow, I mean it takes about a week to go soft enough that we have to stop and get it pumped up. Lately it seems to be leaking a bit faster, so we have to pump it up every 3-4 days. When we try and patch it, we can't find any hole to patch. Could it be the valve? Any other ideas? We actually replaced the tube and found the same problem with the one we put in!

i would look for a very small piece of debris in the tire, such as a bit of radial wire, or some frayed derailer/ brake cable. they can be nearly invisible, and protrude into the interior of the tire just enough to slowly scour a pinhole in your tube. this is a great example of why simply running your fingers blindly around the inside of your tire is kind of nonsensical.

To answer the OP's question, yes, you can have a faulty valve.

I've had faulty presta valves. I usually unscrew the valve core and put a few drops of blue lock-tite on them to seal it all up.

After you pump up the tire put some spit around where the valve core screws into the valve and look for bubbles.

+1 to all of that, but... Needing to pump every few days is normal on most of my bikes. The skinnier the tires the more this is true.

It called Ficke's Law of diffusion. Basically, you have a high pressure system inside the tube and a low pressure system outside the tube. The tube isn't perfectly impermeable to air inside the tube and it works it's way out of the tube through the rubber. This happens with all high pressure systems but metal containers tend to be less permeable and retain air much, much longer.

The rubber of bicycle tubes is particularly permeable with respect to carbon dioxide. Fill a tire with CO2 and you'll be pumping it up the next day. That's why CO2 should only be used for emergency filling (lazy filling) of tires on the road. Fill them with real air before the next ride 'cause CO2 is expensive!

For us, needing to pump every few days isn't normal. Aside from punctures, I think maybe we pump up our tires once every 2-4 months. We check them often but they stay solid a very long time. We are running Marathon XRs.

We have Schraeder valves, by the way. The spit idea is a good one. Will try that next time we're in a hotel room with nothing to do

Two to 4 months is a very long time to go between pumping tires. My tires usually drop 10 to 20 pounds per week at home or on the road. Perhaps you just started noticing them recently. Just in case you have a slow leaker, do a water test and run a cotton ball around the inside of the tire. The cotton will catch on the stuff in the tire and pull off some fibers. Saves your fingers

I only use imported air that I get from the local poseur shop. It's the same air that was used by Lance to win the Tour de Hoboken. It says so right on the can.

I am amazed that anyone gets by with pumping tires that seldom. Even on our touring bikes with relatively fat tires (700X32 100psi) I never go more than a few days at most.

Out of curiosity what size tires are they? Also what pressure do you run and how much do you allow the pressure to drop before pumping them? The fatter the tire and lower the pressure the less frequent the need to pump.

On the valve thing I have never had Prestas leak, but have seen Schraders leak mostly on cars. As was said a spit bubble on the end of the stem is the test. Tightening or replacing the core in a leaky Schrader valve is usually a fix.

"I only use imported air that I get from the local poseur shop. "

You may be joking but my local praxair supplier will sell you a system to pump your tires with some gas that doesn't leak out. I can't remember what they were using maybe argon. Apparently this reduces fuel costs for fleet vehicles.