ATL UM4 Access A probe leaking

I have an ancient ATL UM4 with a leaky ATL Access A, 3.0Mhz mechanical scanhead. p/n 101-24900-66. The scanhead is leaking from the black O-Ring 3/4 of the way down the probe head near the cable where it enters the scanhead. I have the proper ATL acoustic fluid to refill the scanhead (ATL p/n 130-27009-07) . What I don't have is a good sealing O-Ring. I would like to disassemble the scanhead, replace the O-Ring and refill the scanhead. It used to work before the fluid leaked out of it. I am hoping it will still work if I can refill the probe.
Any ideas how to get it apart? It almost looks like the base twists off, but I am not sure.
This scanhead is for an US system (ATL UM4) at a clinic that has no money to purchase a new US system. The image quality of the UM4 was adequate when the Access A probe worked.
Any ideas?
Steve