Transmission downshift issues

So I haven't been able to drive my 65 Malibu a lot lately. I've had it stashed at a storage location that I can only get to once every month or so. I'm actually trying to sell it so I went out to show the car the other day and after cleaning it up and taking it for a spin I noticed an issue. Under normal driving conditions the car was shifting just fine. However when I mashed the peddle to get the car to downshift it would downshift but then seem to lose power. The trans made a strange sound that I can only describe as "struggling". As soon as I let off the peddle everything was just fine. If I pressed the gas down normally all shifts happen just fine. I did notice that I'm leaking what appears to be tranny fluid. Could this be something caused by low fluid levels? I'd appreciate any and all help. Thanks!