Custom validator not persisting its property values during post back

Hi everybody,
i have created a custom validator (extends webcontrol and implememnts Ivalidator) to validate currency amount entered by customer, it has two properties Min  and Max amount. i am setting these values based on their selection in dropdown called SelectRange. i have set autopostback property of dropdown to true. and attached an event handler to selcted index change event and there i am setting the custom validator min range and max amounts based on his selection in dropdown.
after the dropdown selection is made, and once the user tries to enter an amount and click save button, the custom validator is not validating it as expected as the MIN and Max value set to the validator during selected index change event are not persisted during postback caused by button click .
Can you please help me to understand what i am missing here.
Thank you.
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