[Phillyfanfrombirth]

"hissing" is usually a vacuum leak... any chance that you have or could borrow a vacuum gauge to find out where you are?

Are your fluid lines free of kinks?
How's your shift linkage?
Do you have the lines running to the radiator, or an aftermarket cooler?
Is the transmission vacuum line hooked up to a full time vacuum port?

If you have fluid that isn't being impeded... and you have solid vacuum read out, and the linkage checks out then it starts to sound more of an internal problem...