I have a new rebel t3i when i set the flash to manual the flash output level for the built in flash will not go higher than 1/4 power. it will go lower down to 1/128 power but not higher than 1/4. I contacted canon and they replied back that they do not know. I sent a second e-mail and that person said that is the way it works. I don't beleive these two people really know, and all they do is answer e-mails. Somebody must know why this happens. Any help would be appreciated.
I think you're mis interpreting the manual. From reading the t21 & t41 as well as the 7D manual you don't have the ability to set the power output of the built in flash. You do have that available on the Canon Speedlights (ad on flash units) which are specified as fully compatible with your camera. .