They returned shortly after 10.30 when temporary airconditioning units were installed. A health and safety official previously told the ECHO: "It is like a hothouse in there. It is absolutely unbearable.
"They brought us some air blowers but all they did was pump out dirty air. We have to spend four hours a day in there and it is impossible."
The dispute left people living in the L1-L8 post codes without any mail deliveries.
Mail chiefs were today meeting with representatives from the Communication Workers Union in an attempt to solve the problem.
A Royal Mail spokeswoman said: "We are currently putting cooling equipment into the office in an attempt to resolve this situation and we are hoping this is a one-off problem.
"Some mail has already gone out and we are looking forward to the full service being restored shortly."
The walkout is the latest setback for the Copperas Hill sorting office which has been beset by problems ever since the introduction of a single daily delivery earlier this month.
Postal staff say the change to just one delivery a day has left them struggling to cope with the huge backlog that ensued. Last week the ECHO revealed that union chiefs were threatening to ballot for strike action unless the situation improved.
The postal problems in Liverpool over the past three weeks have included people going more than seven days without post, deliveries as late as 6.50pm and letters going missing.
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