What pressures are you seeing and at which ports? Reduction band or reverse clutch pressure taps? Are you seeing an increase in pressure as you increase the engine speed with your foot on the brake? It is likely not the reduction band if you are seeing all forward gears and no slipping here. If you only have no reverse I would look at the the low/reverse clutch and reverse clutch as these are ONLY used in "R". the manual linked at by Charlie_M is so-so, but it may have the proper reduction servo adjustment procedure inside as you said that you changed this in hopes it would help. It may now be out of "spec" and needs to corrected or you will have issues later due to either slipping or dragging of the band during operation. Tell me what pressures you saw and where and whether you see an increase and we'll go from there. If you need anything else such as the servo adjustment procedure, let me know.
Also, whenever one inspects the clutch material, even on a used transmission just about to be rebuilt, the builder wants to see the writing on the material as this shows minimal wear and glazing/burning and can even be used as verification for the new clutches. Don't hope to find the writing worn off as this is a bad sign, especially on a fresh rebuild. However, some clutches don't have writing on them at all, even when new, so don't jump to conclusions if it is missing. This all comes with experience so keep up your inquisitive nature and don't get bummed out by this bump in the road, you're almost in the clear! Good Luck!