you could have lost the vac pump or you could just have a leak..

this is not easy but it can be done.... i would start with a simple golf T
and i would start disconnecting near the brake master cylinder... and see if
i could luck into it... disconnect plug a line and then turn off the engine and see if it goes off.... and if you hit the right system you will have isolated the leak....

odds are its a leak back through one of the doors or in the trunk...