Hi hugoC-3,
How do you setup the custom fields? is it a numeric field?
Are you outputting it as a select dropdown menu in the filter form?
Please try auto_fill_sort parameter, for example:
[wpv-control name="my-field" auto_fill_sort="descnum" ...] for max value
[wpv-control name="my-field" auto_fill_sort="ascnum" ...] for min value
More help:
auto_fill_sort:
'asc' (default) | 'desc' | 'ascnum' | 'descnum' | 'none'
http://wp-types.com/documentation/views-shortcodes/#vf-153286