Dr. DIFF cobra brake/ master cylinder question

bc3j said

You can quickly bench bleed on the car, if necessary, by plugging the two ports on MC and hand pushing the brake pedal. When you can barely move the pedal, the MC is hydraulically locked and virtually 100% sure there’s no air in the MC. Then remove the plugs one at a time and reattach the brake lines.

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That’s not necessarily true. With the master cylinder in the car you shouldn’t actually be able to mechanically bottom out the master cylinder with the pedal and stock brake rod. And if you plug both ports there’s no where for the air to go if there is any, you need the ports routed back into the master cylinder bowls. Otherwise if you do anything you’ll force fluid or air past the seals. And that’s not what you want to do.

Most of the time there’s enough travel to fully bleed the master cylinder on the car, but that’s the reason there’s a “bench bleeding” procedure to begin with. On the bench you can fully bottom the master cylinder out, whereas in the car that shouldn’t actually happen.