This issue started earlier this winter. When I start the car up, if I press the brake pedal with a medium/firm foot, the engine rpms will fall and jump. If I take my foot off the brake pedal or just barely touch the brake pedal, the idle smooths out. After the car fully warms up this becomes less of an issue.

I do hear a rattling/patter/putting sound from under the dash when the car is cold. It goes away or fluctuates when I apply the brakes.

When I step on the brake pedal with firmness, I hear a whooping (air escaping) sound from under the dash.

The brakes perform very well. I don't feel any loss of braking power.

After the engine is off and the car sits for a few hours, the pedal is very firm until I turn the engine back on.

I've replaced the vacuum valve on the front of the brake booster.
I've replaced the vacuum line on the front of the brake booster going to the intake manifold.
I don't have any other leaks.
I've checked/swapped both my ICV and AMM.

Do I have a faulty brake booster?

Thanks,

Mike

Keep in mind that the cold may be stiffening up the flap in the check valve allowing it to leak. In warm weather the symptoms are likely to go away ie. your booster will be fine.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.

Last year I had those symptoms with my 1990 240. I replaced the booster check valve, it's hose and the master cylinder. That fixed it.

Hmmm. I replaced the valve and hose with no success. It was an aftermarket valve and hose though. The master cylinder isn't leaking and appears to be fine.

You had exactly the same symptoms? Fluctuating idle when cold and fine when warm with the brakes applied?

Thanks!

I have a good used one if you are interested post an email address.

Dan

My MC didn't appear to be leaking either. Yes, I had fluctuations with the engine. But I think they can leak internally somehow. And I think whooshing noise is the MC leaking air, although sometimes fluid is displaced where the MC push rod enters through the firewall.
For mine, no more pedal going down and no more whoosh.
I'm just saying...

Probably. In the booster there is an apply valve and a release valve. The apply valve opens with foot pedal pressure and admits atmospheric pressure to the rear chamber. When you take your foot off the pedal the apply valve closes and the release valve admits manifold pressure to the rear chamber, ready for the next application. One or both of those valves is messed up. The woosh you hear is air entering the apply valve and it gets its air from around the pushrod at the pedal bracket.
Get a used booster. Mechanically simple to install, but a back breaker physically.