After pumping both the tires equally, after a few weeks pass, the rear tire of my bicycle keeps on getting flatter day by day, and looses pressure.
I have checked everything and there is no leakage anywhere. Why always does it effect rear wheel?
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If air pressure is dropping, the air is escaping. This is the simplest explanation. Other hypothetical causes, such as molecular oxygen being bound with other chemical substances inside the tube, are much less likely to happen unless you have very powerful enemies to organize such chemical reaction inside your tires without you knowing it. Even that would not explain the rate of deflation anyway.
Oxygen and nitrogen molecules (which are 99% of air) are really tiny and are in fact continuously escaping the tube though rubber's natural pores all the time. But this process is slow, and it would take months to deflate a tube. Anything faster than that means that your tube has larger (but still tiny) holes or a faulty valve.
I have checked everything no leakage anywhere.
Patched holes in a tube can sometimes re-open when temperature gets higher (this is mostly for "glue-on" patches, not the vulcanizing ones). Invisible smallest holes can become visible when a tube gets inflated to a higher pressure. Putting a tube under water and taking your time to monitor all suspicious bubbles on its surface is still the best way to find leaks.
For the peace of mind, I recommend changing the tube to something of a better quality, or at least not from the same manufacturer, in order to make sure that it is not a factory defect affecting the whole batch of tubes. A usual advice is, while changing tubes, is to re-check the inside of the tire for foreign sharp objects. You do not want to spoil a brand new tube due to an oversight.