If a riser breaks off flush with the T-connector, the part of the old
riser that is still inside the connector will need to be extracted
before a replacement riser can be threaded into place.
A folding saw (the kind used for pruning trees) will do the job.
Insert the toe of the saw (the part farthest from the handle) into the
old riser. A lot of force is not necessary, and it is not necessary to
jam the saw down into the old riser. The teeth of the saw will bend
just enough and bite just enough to grip the inside of the old riser.
Once the saw is inserted, start turning the saw. The old riser should
come out quite easily without damaging the connector.
When the saw is only halfway open, it forms a 90° right angle; and
that angle makes it that much easier to spin the old riser out.