Bottom line. I just replaced all my bleeder valves. If yours are old, then you should too.
Years ago I was using “Speedbleeders” brake bleeder valves. They’re pretty cool. They have a checkvalve in them. Turn them a little bit and they turn into a one-way valve. This means you can hook up a hose and jug, jump into the driver’s seat, pump your brake a bit and then just get out and close the brake valve. Don’t need 2 people. Don’t need a pressure jug.
They didn’t last long tho. The first one failed after about 2yrs, and by 4yrs, none of them would do the cool checkvalve thing. So I replaced them with conventional checkvalves.
Tonight, #1 son and I replaced the master cylinder that I’d grown suspicious of at CMP the other weekend. My brake fluid was old so rather then doing my cool MC replacement trick that doesn’t require bleeding the corners, I did all conventionally. Therefore, I had to bleed the corners. And none of the corners cooperated all that well.
In order to get fluid to move I had to loosen the checkvalves so much that they leaked at the threads. And it seemed to require a lot of pressure with the pressure jug to make fluid come out. I think one is only supposed to put 10psi in the jug and I couldn’t get any fluid to come out until it was over 11psi. Remembering how those Speedbleeders failed years ago, I went to the local autoparts store and bought new bleeders.
It only took a moment to install the new bleeders and immediately all 4 corners started behaving. The fluid would come out at a reasonable rate without having to turn the valve so much that it started leaking at the threads.
It’s not clear to me how they quit working. There’s no water in the system to speak of so they can’t rust. There’s no contamination in the fluid to plug them, so the cause of the failures is a bit of a mystery. Generic bleeder valves can be found for just a couple bucks tho, so replacing them is trivial. If you’ve not replaced yours, I would.