I agree with getting your tank tested but still cannot understand why you don't have to considering it can kill just as easily as a small boiler taking into consideration the difference in the expansion of steam versus compressed air etc .
If it were a boiler you would legally have to get it tested and in industry it would be yearly , here we have to have any compressor other than domestic use done yearly also .
You can get it tested with ultrasonics which will give a general idea of its condition but a pressure test is the best .
You also need to get the pressure relief valve checked as it could be faulty .
I know it costs money but i would rather spend the money on maintaining my equipement rather than paying for my funeral or even worse some one elses !
A copper gasket will be easier to make and anneal as you can see when it is red hot .
You can use soap as an indicator as to the heat level in alluminium , just smear it on and heat from the other side – when it ggoes black and burns it is hot enough .
You can also blacken it with the soot from an oxy torch ( don't breath in the soot as it can cause cancer ) then heat until the soot burns off which is close to melting point .
With the leaky mud door it will depend on whether it is a corrosion problem or if it is warped .
Corrosion – i would recommend getting rid of it as it is most likely corroded elsewhere .
Warped – i would be asking the question how it has happened ?
If the door has never been opened it should not be leaking so it could be a warning as to an impending failure .
If the door was opened then why ?
When buying a new unit please ensure that it meets or exceeds your local manufacturing / safety standards as there are some dodgy pieces out there .
Oh and by the way EVERY time you start up the compressor give the blow of valve a tug to ensure it is not frozen – same goes for water heaters give the pressure relief a a tug here and there as they too can flatten a house !
Ian