Thanks all for the replies.
Bullethead - I was flying a B757 - yes we do have a manual mode. Yesterday we were loathe to use it as it has the worlds most sensitive control and you can easily make the rate go from 0 to 1500ft/min in a blink of the eye. After all we were only 0.1 PSI out and inadvertently turning the knob the wrong way could have made an issue into an incident. Our pressure relief valves are doors which blow out and need replacing on the ground - I think they just pop off, but having never seen them in operation I'm unsure. We have two - so there must be a margin there too!