Transmission

Car has been setting for five years but have started frequently. Getting it back to drive and now the transmission gives a gearbox fault. Shift into reverse really well. But fault comes on when you shift into forward. And seems like it is slipping. Transmission was rebuilt before we parked it and worked well. Also comp was rebuilt too. Should I start with checking fluid? There is not a dipstick. I pulled plug above sump on housing and fluid came rushing out is that the wrong plug? Long story why it set so long. But still in great condition and my wife wants to drive again.