My 1962 544, with about 10K miles on its swapped-in D-type overdrive from a P-1800, started to slip out at highway speeds after warming up fully. So fluids were drained and refilled (with red-line MTL), screen and fine filter cleaned (some tiny metal flakes found), unit engaged to warm up fully, and fluid then topped up to assure proper levels in the whole system -- M40 and D. Plus electrical connections tested and re-tested. But the problem is now worse -- overdrive won't engage at all. Would this likely be the pistons in the unit itself, or maybe just their o-rings. My willing and patient mechanic needs expert advice (and thinks I may need to replace the entire unit). Hope not, but if so is there a stash of serviceable D-types anywhere? Thanks in advance for your help. The 544 is my daily driver.