Do you get the impression that the booster is not using the vacuum that it has? The push rod on the nose of the booster that pushes the master cylinder has an adjustment that determines the point at which the booster’s internal valving applies the vacuum boost. Maybe it needs adjusting. Although both boosters I put in my two Concordes needed no adjusting when installed, some people have reported that theirs did need it. You say yours was never great right after replacing it. Maybe it’s adjustment was marginal and something has shifted further in the bad direction so that it’s not just marginal anymore, but never activates.
Not saying that that is definitely the problem, but there are instructions on how to test and make that adjustment. I don’t believe the FSM goes into that, but I think I scanned the instruction slip that came with an aftermarket booster and may have even posted it. I’ll try to find it.
You have to remove the booster to make the adjustment. Since you have to remove it anyway, you could consider buying another booster and trying it to see if that fixes it whether the problem is the rod length adjustment or an internal mechanism problem. Worst case, you’ll have a spare booster and be out $100 (or whatever they sell for these days) if that doesn’t fix it. (It’s a long shot, but you may find that the rod adjustment lock nut is loose so it has changed length - not likely, but check.)
Since it appears that vacuum is getting to the booster, from your description, it would appear that one way or another the booster is the problem. (Check that the vacuum hose connection is tight at the plenum. I was loosing boost, and discovered that the vacuum hose was swelled up like it had been exposed to oil and was loose on the plenum nipple - leaking like a sieve at that connection. Probably not the case since you detected that the booster is getting vacuum - but as you suggested, is it enough vacuum?)
If someone else has other ideas, please post.
'98 LXi - Later Concorde gages (black w/ chrome rings)/'99 LX - LHS gages (white) - HIR bulbs
Last edited by peva; 09-27-2019 at 08:03 AM.