Tube deflates at rest but holds pressure while riding

[nutnelson]

Can you explain this? I put in a new inner tube & pumped it to proper pressure & rode six miles on city streets. When returning to the bike a few hours later, the tire had deflated. I pumped it up (hoping there wasn't a hole) and, again, it held the proper pressure all the way for another 8 miles. After sitting a while the tire again deflated.

So, I've been greeting each ride with a pre-pump for over a week. I'm baffled at why it doesn't deflate while riding, returns to the proper pressure after pumping it up, but deflates again. It's time to put the bike (1980 Raleigh, mixte, 27") on the stand, remove the tire & check the tube.

Can anyone explain this? What do you suggest I look for? (I've checked the inside of the tire for glass, observed the opening in the rim where the valve goes in -- nothing seems out of order.)

[GeorgeET]

Rubber tubes do loose air after a while, but not that quickly. Yes check the tube in a tub of water and check to make sure the valve core is sealing and is tight.

[1FJEF]

You have a slow leak. Pull the tube, fill the sink, fill the tube, submerge one section at a time to slowly inspect & find the tiny leak.

[nfmisso]

if you don't find the hole from the air bubbles in the clear water - add one drop (no more) of dish washing detergent, mix well into the water, and re-check. The soap makes the bubbles much more visible, and helps detach them from the surface of the tube, so they rise straight up, instead of following the surface of the tube.

[1FJEF]

I haven't heard of that one before. Good to know.