If this is the P3 line in the Allison, it has been the cause of a number of engine failures over the years.
There appear to be a number of causes for the failure - two predominant ones are fatigue and maintenance error. The first is self explanatory; the second (and of course the first) can be caused by the frequent connection/disconnection cycles. As JerryC has mentioned, in the absence of an installed valve for the purpose, it is disconnected and blanked for every comp wash. There is an STC for the valve but it is not an 'approved' modification is some States (UK).
Some oil companies/operators replace the P3 valve at regular intervals - this appears to solve the problem.