It takes 3 gallons of ATF to fill your TH400 trans, it will not work
much on less. Also make sure the filter is not plugged, that
cartridge type is harder to find. I use that vintage trans but
convert them to a 68 up filter, available everywhere.
There is is no linkage that will stop the trans, short of not putting
it into a forward gear. An original needs a total refresh, all rubber
seals inside and out, though the hard parts could be just fine and
I would re use them. The factory manual is full of info to do this.
good luck, Bruce Roe