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The primer pump going firm shows the sytem is primed to its full low pressure and as the engine draws fuel into the high pressure pump the bulb will naturally go soft. The sudden drop in the fuel gauge is more likely due to a sudden voltage change (modern car electrics are very succeptable to voltage variations). This could be down to a poor battery or poor earth connection between the engine and the bodywork. You could try making a temporary earth connection from the engine to the negative terminal of the battery using a single jump lead. Just take care - don't get them mixed up as it could make matters worse. I suggest you check the earth connection from the engine to the bodywork is in good condition - give it a tug to ensure it is sound.
Currently in madnoel10's garage:
Honda Civic 1.4l