I find the old used ones are corroded. I think a bit of brake fluid and some moisture gets up in there over time and it corrodes. I took a few old ones apart hoping to be able to repair one.
They develop a white slimey crust that stopped it from working. Then once they were apart the way they rolled the metal over at the bakelite end makes it pretty much impossible to get them back together. These were all old used ones that were on vehicles that hadn't been on the road since the late 70's to early 1980's so I could rule out Chinese and Mexican inferior parts.
They seem like a crapshoot on how well they work. The one on my 63 Econoline only came on it I was really stepping on the brake. It was a new one in 1998 - 99. It's now dead as well. On the bright side, that one never came on at random.
Here's a NOS one on epay for 15 bucks plus shipping. Might be worth a try.
https://www.ebay.ca/itm/263321578012...UAAOSw-olaCmig