Most Air Transport & Business Jet Pitot Static systems do not have a drain port as they can too easily be compromised by rain & atmospheric humidity. They have separate Drains with bottles that need to be checked & emptied if needed. Even then, the amount of moisture especially in the tropics can cause problems such as those that gave rise to this
AD. The sizing of the total internal volume was, from memory, a bit of hit & miss in that it was a compromise between having sufficient volume not to be overwhelmed and being too big such that there was unacceptable lags introduced into the Air Data System. (As an Avionics Engineer I was not involved with this - it was a collaboration between our Flight Sciences Group and the Mechanical Designers).
By the way, most Aircraft Manual illustrations (such the one above) are much simplified as they are for general system understanding and do not include the structural details contained in the Maintenance Manuals. In addition no modern jet will have directly plumbed instruments as shown - the Pitot Tubes & Static Ports will connect to the Air Data Computers or, more recently, the electronic sensors are contained within the probes themselves. Similarly the Standby Instruments have not been mechanical on most large aircraft for some time and read out on the Standby Attitude Indicator.