Did you have a complete trans to start with?
I have been building race 904's for 40 years and if you piece a 904 together a 904 the problem is there are 6 different combinations between the 904,998.999.and the 518 that all look alike but the passages for oil flow go in different directions and they use different sealing ring. All parts need to be matched. The sealing ring on the input shaft are even different sizes. Check all that out. If you are still having problems call me. 563-349-7412.
Thanks
Jim Petersen