Map is not matching correctly against upstream custom header

[Gabriel Arrais]

Hi, I'm trying to configure my proxy cache settings based in a response
custom
header, using proxy_no_cache and proxy_cache_bypass directives.

First question: is it possible at all?
Second question: If it is, why this map is always hitting the default
value?

   map $sent_http_x_my_custom_header $no_cache {
        default    0;
        1          0;
        true     0;
        false      1;
        "~*false"  1;
    }

    ....

   location ~ ^/ {
       ...
       proxy_no_cache  $no_cache;^M
       proxy_cache_bypass  $no_cache;^M
   }

I've already tried the map with $sent_http_x_my_custom_header,
$upstream_http_x_my_custom_header and $http_x_my_custom_header. It's
always the same result.

obs: I've already tried with if but if is resolved in request time so it
didn't work.

Thank you in advance.

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