Must admit to looking for some kind of vent months ago. Expecting to see something like the Ford axle one with a cap on it. Looking at the manual for the Rapier, it is just a 3/32" hole in the top of the casing between the 'u' bolts and the 'banjo' diff case. Cleaned and checked now. jobs a carrot. Cheers for that.
Perhaps you already have done it, but please clean the bleeding bolt on the differential, my brothers Alpine had a blocked bleeding bolt which caused the pressure including the oil to escape where gaskets and oil seals are situated.
At least we don't have to keep filling the diff and cleaning the brakes and tyres!! Other side went better with less colourful language. Got it in the press at ten ton and gave it a whack with a copper drift on the side of the hub. Popped in no time and no damaged thread to repair. pleased they are sorted, car can be used more now.
Welcome to the "club"! I have done this job a few months ago on the Sunbeam Alpine of my brother and like you I have used a lot of words I don't want to repeat here. My experience was about simular to yours and I also had to repair the thread on the shaft.
Set off this morning with the idea of changing both rear hub seals on the series car. Yeah right! After trying a standard 'wind it' type puller and whacking the end, as per workshop manual, we went for the 10ton hydraulic type with no joy, things were looking grim. We ended up taking the whole halfshaft out complete with backplate and bearing. Can't get under the hub on the normal bench press so had to smash off the outer case of the bearing and take the brake backplate off the wrong direction. (Good thing I own a bearing stockist business.) We could then get under the hub. It's on a taper with a keyway. Must have been cheaper to manufacture than splines? We had heat to it and 20ton showing on the meter before it eventually gave up and came off. Slight damage to the thread on the shaft so to the machine shop to tidy up. What should have taken an hour ended up being four and a bit of name calling. Car was called a few things other than a Sunbeam!! Worst part is we still have the other one to do...........