The only time I experience EXACTLY what you are experiencing. Is when the brakes are cam'd over. I'll push the pedal down. Nothing happens untill 15 psi. Then BOOM. Insta brakes. There's NO easing in to them. Meaning, the brakes are at the top of the cams. Because they're either worn as far as they're going to wear. Or the drums are worn out. Usually indicated by a nice sized lip at the edge of the drum surface where the brakes ride. All it takes is ONE brake or drum to be worn out to experience that problem. And I have 14 brakes. The fact that your adjustment isn't holding is another sign the brakes are cam'd over. Did you change your drums with the new brakes?
If the adjustment isn't holding. That's where I'd be looking. It's possible your brake springs are broken or missing also.
You're right, one is broken, the parking brake 33.3 spring has a crack. Can that cause these results?
It's possible. And you need to find out why. They don't generally crack and break for no reason at all. I've always ended up with broken springs with cam'd brakes. Not saying that's your issue but it sure sounds like it. You have all the symptons I get.
check the pins that hold the break pedal to the valve .I have had them freeze up . an have no break at low pressure , then when you push harder it almost locks up the breaks
Copy. Come to think, that chamber was replace with the old drums, relatively 6-12 months back. The rest were replaced with the new, so it sure sounds you're right on the money. Trying to save me a few dollars, seeing it was recently replaced, my mechanic didn't replace that chamber. Later, I had the dealership inspection, they caught that chamber with a cracked parking spring.