I am considering just changing to the upgraded pressure valve, but am unclear as to if this can be done without removing the valve body. It looks like removing the valve body requires first removing the transmission and inverting it.
I would appreciate info from some one who has done this, or at least had the transmission apart.

 

I haven't done it, but intend to in the near future. The valve body comes off from the bottom after removing the pan and filter. The valve body has to be removed to get at the valve seat (the cover and spring screw into the side). I understand that the valve piston itself can be replaced if the bore isn't worn, but the better way is to ream the bore and use an oversize piston and new spring. There is a kit available for this. There is a long thread with details posted not too long ago. There's also a YouTube video on a European ZF repair facility re-doing a 5HP24. Hope this helps. Evidently there is a ZF distributer in CT that can help you out, they may have redone valve bodies.

 

I watched the video, in which the trans. is inverted on the bench. He cautions that you have to be careful that an o ring and spring do not come out. If the trans. were still in the car there would be nothing holding them in unless they could be placed in the valve body.
Without having one to examine I would be hesitant proceed.
Gus has responded to this thread on the XJ8 forum and will be talking to ZF today. He will get back with more info.
If it's possible to just replace the valve and avoid future damage that would be a huge saving. Even having to remove the valve body and reaming the bore to install a oversize valve would be a big savings, if it could be done with the trans. in the car.

 

I can't help but am interested and appreciative of whatever you learn. At some point relatively soon I want to do this and have my A-drum replaced preventatively.

 

The discussion on this and the XJ8 forum is leaning towards drum failure being a symptom of the valve problem and not an inherent fault of the drum. What I take from that is it may not be necessary to replace the A-drum as preventive maintenance if the valve body is not worn, or is repaired if some wear is present.
The key is whether there is a lag after shifting into a forward gear before engagement. Gus said he would ask the ZF techs and get back to us.