mod_proxy_balancer: Passing heaaders from backends to client

Hey, guys ..

I have a setup where there are two backend servers reverse proxied by
a frontend running mod_proxy_balancer.
The backend servers send fine caching headers like "Expires", but
these seems to be dropped by the frontend, as they are not part of the
reply between proxy server and client.

How does one normally resolve this situation ?
I have tried to find a way to tell Apache to pass through selected
headers from the backends through the proxy, but I can't seem to find
anything in the documentation or on Google ..

That would be my preferred way of getting those cache headers back,
but for now it seems my only possibility is to configure the caching
directly on the proxy server, using mod_expires or something
similar ..

Isn't there any way of telling Apache to pass though headers from the
backend when it is acting as a proxy ?