1968 Chevy Camaro Rear pinion seal leaking

Drive Train Axles Bearings problem
1968 Chevy Camaro 00000 miles

I had a professional rebuild and replace the ring and pinion in my car. Since then, after driving the car, a puddle of rear end fluid ends up on the floor under the rear driveshaft yoke. I have replaced the pinion seal three times, but the problem persists. It only does it after a lengthy drive and does not leak while driving. I checked the differential vent, but it appears to be clear (blew compressed air in while rear cover removed). I am at a loss for the cause of this.