Why don't parts like billet kickdown servos seem to exist for them if they're such "great" tranny's?
I know sonnax used to have them but apparently no more.
Working on a 91 a500 out of a 318 Dakota (not what it's going back into)

The fact the parts you are dealing with are nearly 25 years old could be part of the problem.

Also, I believe the 904/A500 are now referred to as R42 transmissions (but I could be wrong). Having the correct current description of the transmission will also help.

I don't know why I could find what I'm after easily 5 or so years ago but now it's unobtanium.
If I'm replacing the rear servo and the accumulator I don't want to "not" change the front servo with similar parts.
And yeah.. on calling a spade a spade"
The last most recent trans I did was a 46re.
I called a local cores place looking for a front pump and was tot"sorry we haven't had any of those cone in in quite a while sorry ".
I stopped by after work anyway and talked to someone else and was asked "you mean 518? I have plenty.". And left there with what the person on the phone earlier said "sorry" to.

I've worked on serveral Pro-Flites recently. The cream of the crop Torque-Flites. They are had factory servos. Think about that.
Doug