First of all
check if both front and back tyres behave alike i.e. Apply full brakes and check the marks on the roads.
If only two marks are present (front wheels) then your rear brake drums needs work. Which includes sandpaper buffing to tooling of the drum surface.
The rear drum is very common case with ALTO.
If this is not the case, then get master cylinder checked and serviced.
Still doesn't works, then turn to @kiani's solution.
BE CAREFULL.
There are many ALTOs accidented just because of this rear drum fault including mine. And i need it to get serviced every month for 3 months now.
BTW ALTO manual also says to get these inspected/service every 5000 Kms 