My old bus and Mrs Bagshots Freeby both had the same issue. Tank and caps looked fine but when the hot water got to them, there were tiny crazed cracks around the spiggots that couldn't be seen when cold and it seeped through.
New tanks and caps on the 'bay for less than a tenner and smartens up the engine bay a bit.
New tanks and caps on the 'bay for less than a tenner and smartens up the engine bay a bit.