MG Car Repair: MG Midget radiator expansion fill-tank, overflow pipe, radiator cap

Question
I don't think my Midget expansion tank is working. The car water system is OK. I've cleaned out a considerable amount of rusty contaminated water from the expansion tank and the small plastic pipe inlet is blocked when I put a screwdriver down it. I was going to drill down it but then though there may be a valve in the pipe. Then I thought the water has to flow both ways so no valve, and the real valve is the pressure radiator cap. Can I clear out the plastic tube to make a clear passage to the bottom of the plastic tank? Thanks.

Answer
Your expansion tank has 2 pipes, one at the top and another at the bottom.  The top pipe is the air release and overflow pipe, the bottom one should be connected to the radiator header tank.  Yes, they should both be clear, but I do not think you should put a drill down them, it might do some damage.  Best to clear them with a piece of soft wire and blowing down them.  Then wash and rinse repeatedly until it all looks clean.

The expansion tank is not intended for filling the radiator.  It should be kept half full, up to the top of the strap and no higher.  You need that air gap in the top of the tank.  To refill the radiator, you should disconnect the large top hose between the radiator and the thermostat housing.  

Normally, plastic expansion tanks fail when the plastic goes brittle.  Either they split, or one of the pipes breaks away from the tank.  They are not repairable.  Personally, I would get a brass replacement tank.