TH400 lost reverse
#2
Might be a broken rear band. Or reverse servo problems, you can inspect the rev servo by removing the pan, servo cover and the servo itself. You might get a check on the band by putting a screw driver in the servo pin hole and making sure the band gets tight when you push, otherwise you need to take the trans apart quite a ways to access it.
#3
The quickest way to check would be to pull the VB and apply air to the reverse servo feed. A thud and hissing or just hissing is a servo seal problem. A loud bang is a band problem or short servo pin. Might even have a broken servo piston if it's been bottoming out due to a broken band.