Just fixed a 91 with the same issues. Check the pump with direct 12v to the terminals to verify it’s the pump itself. Probably a bad pump. That said the system doesn’t need the pump to activate the secondary’s. Engine vacuum is enough. The pump is there to fill the gaps like when your on full throttle and engine vacuum disappears. The likely culprit is the check valve under the plenum, barring a major vacuum leak elsewhere under there.
Here’s a good write up. Not sure who wrote it.
The test of that Vacuum Check Valve is a simple test without removing the Plenum. Just disconnect the Plenum Vacuum line on the drivers side and hold thumb over end of disconnected line. With the Ignition Switch on the Vacuum Pump will keep cycling until you plug the end of that vacuum line fitting. The Vacuum Pump will then stop cycling indicating no Vacuum leaks unless of course you have additional Vacuum leaks in the Secondary Vacuum System. If disconnecting that Vacuum line results in the Vacuum Pump not Cycling without blocking with finger, the Vacuum Check Valve is operating correctly. The Plenum has to be removed to replace that check valve.
H
