The first thing I would look for is the vaucuum lines under the hood, check all vacuum lines especialy the nylon ones. Second look at the plastic check valve near the intake manifold where the nylon hose goes into, my 88 760tic has two of them, take them off and clean them out and check them, you should be able to blow in one end and not the other. They are pretty expensive to replace, $27 at the dealer, so if one is bad you usually can clean them out and get it working again. Third check all vacuum lines underneith the dash, you can check all of them with a vacuum pump at the control switch(passenger side, underneith the glove box) and check all the servos to see if they will hold vacuum. (servos are on the drivers side near the accelerator pedal)

Good Luck