That's probably just a bad case that has never been cleaned. I had my filter apart today and it was just a complete a*se of a job. I'd decided to do it as a precaution as it hasn't been anywhere since 1994 and I am coming to the end of the rebuild (just the reversing light, and washer pump not working, & a new horn needed before MOT)
The filter wasn't too bad & cleaned up very well, but both the nuts on the studs on the drivers side of the rear panel sheared off :evil: but the other side was fine. I then had to grind & drill the studs out & then drill completely through the panel using the stud holes as guides. I then dremelled with a small cutting disc, the holes into squares so I could get a socket into the box section, and put it all back together with bolts. I then made some covers out of inner tubes for the square holes. Then I could not get the engine mount to bolt back together beacause I realised the thread had collapsed on the mounting plate

My eventual solution was to unbolt the mounting plate and unclip the plate and then turn it upside down so there was 'fresh' thread for the bolt to attach to. It seems to have worked. Haven't put the bumper or bonnet lid back yet though...
What a way to spend the Bank Holiday. Starting at 10 and finishing at 6 & cleaning the filter only took 15 minutes. Don't you just love old cars, but then that's what it's all about :wink: