The vehicle in question is a 1989 GMC Jimmy 4.3L 4x4.
When the brake is pushed, fluid squirts up from the primary through the reservoir. Not just a little, and not just at first, but for the entire stroke. Under hard pressure, it’s a jet. The secondary produces no backflow whatever.
I used to think there was some check valve between cylinder and reservoir, but everything I can find says “no”. It’s an open port that’s supposed to be covered early in the stroke when the piston seal passes it.
The fluid backflow is constant throughout the brake stroke. Judging from splatter, it’s at least 3-4 oz fluid volume, which I’m guessing is a large fraction of the volume needed to actuate the brakes. The pedal goes to the floor with only modest resistance (engine off, so no brake boost).
There is some air still in the lines, but the more they are bled, the WORSE the blowback gets. I could see maybe compressed air squeezing past a crappy primary cup and decompressing behind it, but the sheer volume of air necessary is implausible.
Over the last few decades, I’ve seen this kind of behavior dozens of times. It is almost always correlated with weak braking. When this particular brake system failed, I promptly observed tragic blowblack through the primary. It did NOT do that when it was first installed.
Not coincidentally (I think), the gravity bleed through the primary is much slower than through the secondary. The primary fills and drips so slowly that it might be only filling by leakage past the secondary piston seal, not the compensating (?) port.
I tried opening the piston slightly (1" pedal depression, as gauged by my wife). On a previously gravity bled cylinder, this produced some air bubbles (mostly but not entirely through the primary). After ~20 strokes, the air stopped and some slight backflow (through the primary only) was seen. Also, there was slight backflow through the secondary when the pedal was /released/, while the primary reservoir showed a slight draw-down as expected. No improvement in performance. The eventual remainder of the stroke showed equal blowback.
I’ve tried six master cylinders of four brands from three vendors. The behavior is essentially the same, although some were more extreme than others.
No sign of leaks anywhere else in the brake system (after repairing a few).
The brake pedal goes to the floor even without power assist, and past experience says this magnitude of blowback through the reservoir is always a problem. A thorough four-wheel bleed might change things.
There is something I’m missing. I’ve searched and seen other people report the same thing a few times on other forums, only to be ignored or evaded. On other vehicles I’ve had, the issue has always been resolved by replacing the master cylinder. I’ve never before seen this kind of blowback from a new cylinder, or even a rebuild, and I’ve installed a couple of dozen.
