boost problem

[PureButeS12]

hi all, last week my S12 stopped boosting all together, it wasnt even registering negative boost, but after a quick look under the bonnet i found one of the vaccum hoses had split (assuming because of age) so i repaired it

it worked fine for a week but now its stopped boosting again

this time the boost guage is still reading negative boost as normal but won't go above zero when driving, so am guessing this isnt another vaccum problem cos the guage would of just stayed at zero on tick over

the boost pipes all look secure, actutor arm isnt stuck, i think the dumpvalve is fine too, could this be a problem with the safety valve on the intake manifold or something else???

any advice would be greatly appreciated