If all threads in a httpd child worker process are saturated with long requests, then all connections in the pool are exhausted, likely leaving none available for additional pings, which then fail with errors like:

[error] (70007)The timeout specified has expired: proxy: ajp: failed to acquire connection for ...

Documentation suggests the connection pool will be sized to ThreadsPerChild+1 to avoid that, but it looks like it is really just ThreadsPerChild on httpd 2.2.