Brake booster less noisy after unrelated maintenance?

hmmm....

 

ChatGPT has clearly been trained by reading terabytes of the previous generations of internet hogwash that tries to hold your attention as long as possible without you noticing it's got nothing to say about your problem.

There isn't supposed to be air in the brake system. The brake booster pump (which is a separate part from the brake booster) doesn't pump air, it pumps brake fluid. It pumps that fluid into an 'accumulator', which does have some 'air' (well, nitrogen, about 78% of air anyway) trapped in it, on the other side of a sealed bellows. That squeezes the nitrogen, making the pressure to operate the brakes.

The nitrogen is supposed to be trapped on its own side of the bellows there. Sometimes after years of use, the bellows might leak, and then there will be nitrogen bubbles in the brake fluid, and the result is much the same as if somebody did work on the brake system and left air bubbles in it (only it can happen without anybody doing work on the brakes).

When you have bubbles in the line (whether they are air from poorly-done brake work, or nitrogen from a leaky bellows), you'll notice the same way. When you press the brake, instead of only enough brake fluid moving to press the pads against the rotors, there has to be a big squirt of fluid to compress the bubbles in the lines and then press the pads to the rotors, and you can hear that psss sound.

Not sure why you're hearing it less now, but none of the other work you have done will have fixed it. You fix bubbles in a brake system by bleeding the bubbles out. That's what you have to do, no matter if they are there from previous work letting air in, or a slow accumulator leak letting nitrogen across.

If the bubbles were only from past work letting air in, then your brake bleeding will be once and done. If the bubbles are nitrogen from a slow accumulator leak, you'll eventually have bubbles again, but that could take quite a while, and meanwhile your brakes work.

Notice that the longer you drive around with bubbles and don't bleed them out, the more cycling you are putting on the accumulator bellows and the pump, because that much more fluid is used every time you brake. If the accumulator already has a slow leak, all the extra cycling will only wear it out faster. If you get the bubbles out now, and you don't need to do it again for a couple years, you may delay the need for eventual replacement.

Also, with big enough bubbles in the system, you may risk a situation where if anything causes the brakes to switch to fail-safe mode (where it's just your foot, the pedal, and the fluid), your pedal will go to the floor and the car keeps going. Nobody likes that.

Bleed those brakes.