Rear brake problem after Q45 front upgrade

[SaGang227]

Ever since I upgraded my front brakes to Q45 calipers, my rear brakes seem to be way hotter than front.
After driving, I can feel the heat at rear brakes and wheels are very hot, while front brakes are cool enough for me to touch them.
At first I thought it was just the heat, but something is wrong because my brand new rear pads are worn down to metal in 4 months.
I do not think that both rear calipers some how seized up at the same time because they turn in smooth when I turn them in and there is no rust on cylinder wall.
It cannot be parking brakes either because they are loosened up and when I look at the caliper, they are fully retracted.
I am guess it could be my master cylinder not working but I don't know how master cylinder failure can cause this.
Also, it's strange that it started after I upgraded my front calipers to bigger unit.
Thank you for your help.