My hybrid bike recently got a piece of glass through my rear tire and innertube.
I bought both a new tire and innertube (700x35c schrader).
I filled up both tires to 65psi. A week later the front tire had 57 psi but the rear tire was at 30. The same thing has been happening for a couple of weeks now.
What can cause this? Did I just cause a small hole while installing the innertube? Is the solution to just replace the innertube again and hope it doesn't happen in the future?
A possible (even likely) cause is a leaking valve. The simplest thing to do first is to see if blowing air out if the valve (by depressing the pin) solves the problem. You wouldn't see the dust if it was there; the only way to know is to inflate the tire and wait a few days.
If that doesn't solve it, or you are less patient than that, go straight to the next step: remove the tire and tube.
When removing the tire, use some chalk or some other means of knowing how it was mounted on the wheel (regarding the valve position and which is the drive side). With the tube, also take notice of which way around it is on the wheel. Now, if you find a hole in the tube you know where to look for the cause in the tire.
Remove the tube and check for the leak by immersing the entire tube in a tub of water.
Be patient, because the leak is slow.
You could have pinched the tube when replacing it (common, but usually leaks faster). But more likely, you still have some glass in the tire. Line the tube up with the tire and check carefully. Patch the tube.
But I really suspect the valve. From my limited experience with Schrader valves they a more prone to leak, especially if used without valve caps.
Other possible causes are a foreign body onside the tire or something (eg a spoke) protruding from the rim.
The other users have hit just about all the points except one relatively common cause: your rim strips may have shifted thus exposing the sharp spoke holes on the inside of the rim, puncturing your tire. You can check for this by submerging your inner tube in water, and looking for any place that you see bubbling characteristic of air loss (you should always do this anyways). If the bubbling is on the inner portion of the tube, it would indicate a spoke-hole puncture. Replace or repair your inner-tube, and replace your rim strip if this is the case.
