When using:
<VirtualHost localhost:7080>
ServerName localhost

ProxyHCExpr world

ProxyPass /test balancer://testAP/

ProxyPassReverse /test balancer://testAP/

<Proxy balancer://testAP>
BalancerMember http://localhost:8080 hcinterval=3 hcmethod=GET hcexpr=world hcuri=/hello_world.html
BalancerMember http://localhost:8081 hcinterval=3 hcmethod=GET hcexpr=world hcuri=/hello_world.html
</Proxy>
</VirtualHost>

Request to http://localhost:7080/test/URL will return 503.