Over pressure
The B747 overpressure valves have clam shell outer doors that can be seen on the left forward fusalage below L2 door.
The valve its self will open at a diff. of 9.4 psid. and close as the diff.pressure falls, but the doors will remain open. They will have to be closed manually on the ground.
The rubber jungle will not happen with a high diff. which will give a low cabin altitude, but with a very low diff. which will give a high cabin alt.