I struggled with this problem for a while, and ended up doing a comprehensive teardown to locate the failing component. You can read about what I did here:
The bottom line, since you don’t really want to read two hundred posts, is that the problem was resolved by rebuilding ore replacing literally everything, which isn’t a helpful solution. The key takeaways: there’s only one vacuum hose that can cause brakes to lock, so for three bucks of vacuum hose, get it out of the way first. I agree with the idea that brake grease may have something to do with it: I cleaned all the old grease out of the reaction valve, and reassembled dry. You want to check the little piston at the front of the master. On rebuilding the booster, a few points: There’s a little spring supplied with the repair kits that’s shorter than stock, so reuse the original. When assembling the master and servo, I used brake fluid for lubricant rather than grease. And the only other oddball thing I did was to run a thin bead of silicone grease around the parting seam of the servo, to prevent air leakage around the edges of the diaphagm. And that’s all I did that differed from the books. When everything was back together, it worked fine and continues to work. I sould add, that all this work was done on a relatively new servo. This diagram may help:
Zeus brakes are a wild goose chase, they fix no problem you have. And if Joey has a magic fix for the servo problem, we’ll probably get it when he tells us about his magic solution to the five speed problem.