DEAR ALAN
I do not know your system however fluid flywheels in buses had centrifugal clutches which were similar to a Honda 50 and were a set of 10 brake shoes arranged in a circle and restrained by a spring until the flywheel spun fast enough to get them to spin out and grab the surface of the fluid flywheel/ tourque converter --- usually car converters are sealed welded units and they can be repaired however you need a lathe to weld them on -- the units I worked on were bolted together and you made your own giant o ring by gluing 6 ft of o ring rubber tighter after replacing the bearings --you were all set then for 6-10 years of trouble free service .
let me know is your lock up clutch internal or external to the converter. regards bugs.
something else occurs to me --in some boxes you can take off the sump and do a resistance check on the solonoids while operating and you could have the box trying to engage a gear on top of a gear not releasing --the solonoids only allow oil to flow in a circuit and also some boxes have test nipples on the out side where you can screw a hydraulic gauge to test pressure of oil in the circuits -before you remove the sump -- you can in some circumstances take the plug off the box and manually send a 12 volt supply to each solenoid in turn and check their operation --you should hear a quiet click from each and a release click also with the sump off . regards bugs