Velvet Drive slipping in forward

I recently rebuilt a 71c transmission.  Was working normally but was leaking from the input shaft seal.  The input shaft was pitted.  I had a machine shop put a sleeve on the part of the input shaft that the seal touches.  I broke the transmission down enough to take the input shaft to the machine shop but did not do anything with the clutch packs/drum.  When I reassembled, it would go into forward and reverse but it slips under load (going much more than idle speed).  Using new oil.  Oil level is good. No burned smell.  Forward, N, and R pressure are all about 132 psi. I have the transmission back out and have it down to what I did to take the input shaft out and cannot see anything obvious.  I'm thinking worn clutch packs but the transmission wasn't slipping until I "fixed" it.  Would appreciate any input.  Thank you.

They far more often slip in reverse than in forward as an early sign of problems.  What oil did you use when you re-filled?

Assuming it's only slipping in Forward like the title says, did you have the big convex forward clutch spring out and  is there any chance you put it back in upside down, which would make forward clutch engagement kinda tough as compared to the spring being in right.

Appreciate everyone's help.  I think I found the problem.  I noticed the snap ring holding the forward piston in the drum was not in the groove.  The forward piston had backed out of the drum just a fraction. The groove was exposed but not enough for the snap ring to fit. There was too much play/clearance in the clutch pack.  I took out the forward clutch pack and several of the fiber disks looked worn. I replaced the whole forward clutch pack with bronze friction plates and new steel plates. Got the forward piston pressed into the drum and the snap ring in the groove. Rest of transmission cleaned and reassembled. Plan to pressure test tonight and check for leaks. Can't really test for slipping until I take the boat back to the lake.  Will be back with an update after I test it. 

I want to give thanks to Keno on giving that link on rebuilding a velvet drive tranny.  My boat has always had that “Dirty Neutral” that drives me crazy ever since I brought her. And I been hesitant to rebuilding and I didn’t want to send it out for I know I can do it.  I was taught to get your hands dirty and learn as you go, and I’m still that way. I attempted rebuilding an automotive transmission a long time ago on my first car and it didn’t go well. I tried reading from transmission manuals on rebuilding but they don’t show and tell you everything you will encounter along the way. And the tricks and tips to boot. But that was before the internet and You Tube. I’m confident I can do it now thanks to the videos. I have some major catching up to do so that project probably won’t happen till next winter if all my other projects (other than the boat) I have going on goes as plan.