that depends;
if the pedal was all the way up, then yes the atmospheric valve may be broken .
But it would be normal to hear the valve working while the pedal is moving, especially while the engine is shut off. In this case, what you are probably hearing is atmospheric air moving into the atmospheric side of the booster.
Here is are the booster tests;
1) with the engine off, pump the pedal several times to empty the vacuum chamber; then press down with a modest force, while you start the engine. As soon as the engine catches and runs, the pedal should fall away from your foot towards the floor, as engine vacuum evacuates the vacuum chamber. This is normal.
2) with the engine running, press down with modest force, then shut the engine off being careful to keep your foot in about the same place. You should not hear any hissing, and it should require the same force to keep the pedal in that position, for several minutes. If not, check your one-way valve and the connections to the booster/intake. (BTW that hose is a special thick-wall vacuum hose; fuel-line will NOT cut it.) If the check-valve is good, and the hose connections are tight, then repeat the test but this time, press down fairly hard with both feet, then shut the engine off. The pedal must remain hard and the pedal must NOT fall away. If it falls away, the M/C is probably leaking internally.
You can prove this. But you will need a helper.
The M/C leakage test;
This test may be messy, so cover everything with rags and especially any painted areas, as brake fluid attacks paint. I will NOT buy you a new paint-job.
Ok so, engine off, pop the top off the M/C. Have your helper slowly, gently, and continuously, press on the brake-pedal. Here is what you should see; for the first bit, the C-ports will release fluid back into the reservoirs, so the fluid will roil just a little. Then as the C-ports are closed off by the passing pistons, all movement in the reservoirs should cease. If you continue to see roiling AND/or a rising liquid level, in either reservoir, the M/C is bad.
When the brake pedal comes to a stop with modest pressure, the pedal should be hard, and it should only be a short ways down the stroke, probably less than halfway down. When the pedal stops, it should remain hard, and not sink any lower.
Now, with the pedal down, put the M/C lid back on and secure it, before releasing the pedal. The reasons for this are; 1) when the pedal returns to the top, the C-ports will be exposed. If there is any air in the system, it will now decompress and blow fluid back into the reservoir . I have seen that geyser hit the underside of the hood! And when it does, it never comes back straight down! And now you are scrambling to clean it up ..... cuz it eats paint. and
2) when the fluid goes out to the brakes and pushes the shoes out, it also stretches out the return-springs. When your pedal returns to the top, those return-springs will force the fluid back into the reservoir thru the C-ports; and you guessed it, this can cause a geyser. So put the M/C lid back on BEFORE the pedal comes back to the top.
When the system is working right with the drum-brakes adjusted up, and NO AIR in the system, you will NOT get a geyser; only a little roiling in the bottom.
