glueball,
It all depends on the circumstances...and that's what we don't know: the circumstances.
What I can tell you from having a hydraulic failure or two in the DC9/MD80 is that, on landing rollout, any brake accumulator pressure from the failed system would be rapidly depleted during the landing roll.
On the DC9-30 where there is a brake selector that allows isolation of either brake hydraulic system, the failure procedure calls for putting the brake selector to the OPERATIVE system, thereby conserving the accumulator pressure in the failed system for later use, if needed ( reselection of the failed system would be required ).
The airplane in question is a DC9-51...not a -30. I don't know if the -50 series has the same brake selector as the earlier series ( the -80 series does not have this selector ), so it is hard to comment with any accuracy.
Perhaps someone with time in the -50 series can comment.