Is that one or two master cilinders?
If this problem was not there from the beginning you could check the rear brake whether it functions 100%. Also the chair brake could be sticking a bit, which makes it come in sooner.
If this discrepancy was always there though, it can be that your chair brake is a little overdimensioned compared to your rear brake. If so, the easiest way out is to put a
proportioning valve in the brakeline to the chair (a neat way way to make your chairbrake adjustable for different loads and conditions btw). Or, in the case of 2 master cilinders you could opt for a slightly smaller master cilinder for the chairbrake, or a brake bias-set.
On the other hand, if you never noticed it and it has never troubled you before........