| Park brake On inhibits normal braking only if actual brake pressure > 35 PSI.
If A/C rolls away, you probably have no more braking, you will instinctively press the pedals and it will work. But there is no way to check actual brake pressure hence the procedure (old MOD), PARK BRK OFF to recover normal braking.
On newer A/C and modified one, if PARK BRK ON and A/C starts moving, press pedals to recover green braking.
If you lose yellow hyd (even LO LVL but depending where the leak is, cf. story below), yellow fluid is trapped between accumulator and available for PARK BRK or ALTN BRK w/o ANTI-SKID thanks to shuttle valves.
It is difficult to answer your question precisely. You have to consider first your A/C version and MOD.
I see only one real case where they lost ACC braking, alternate braking (and normal braking...!): they landed uneventfully, vacated.
While taxiing, G HYD LO LVL. Stopped the A/C on park brake. Whille carrying out ECAM...Master Warning: G+Y HYD LO LVL. The a/c starts rolling, they had nothing to steer the A/C except ENG thrust and one REV and no braking means whatsovever. They luckily steered the A/C on the grass to stop it.
What happened: on (normal) touchdown, left MLG braking unit shattered creating a dual leak in both G+Y hyd lines. They have been able to brake normally during landing roll, once they vacated, braking had worsen the leaks, sensors tripped the ECAM, G first then Y couple of minutes later.
That's a case where you lose HYD at the worst place, just before the brakes.
I digressed a bit just to show you how the weirdest thing can happen, even on a bus! Your initial question SimJock reminded me that event. |