I am not an auro trannie guy, but if it shoots a stream only when in low, it must be forcing pressure in a place that has a leak. I would put it up on stands, start the engine, put in in low, and crawl under there to locate the spot, then replace gaskets or O rings to reseal the leak.
I agree with Ole Don, check the threads on the low revers servo piston cover, and look
for any outside pressure tap plugs some times the tend to leak from being overtightened and split the case.you may have to use some fluorescent dye to find the leak if it isn't too noticeable.
Fordestes