Probable vacuum leak from brake booster; alternatives to replacement?

[blalor]

I think I already know the answer to this, but I'm looking for confirmation. :)

I just bought a '91 325i. The car idles rough, there's a hissing noise in the cabin that goes away when the brake pedal is fully depressed, and the idle smooths out a bit when I do so. The brakes operate fine, although it feels like the pedal movement gets easier after the first bit of travel, not quite sticking at the top, but almost like that. This leads me to believe the brake booster has a leak. I've looked at a couple booster tear-downs on youtube and it appears to me that the only way there could be a leak is if the membrane inside the booster is compromised. The parts diagram https://www.realoem.com/bmw/enUS/sho...diagId=34_0703 shows a replaceable air filter but nothing else that looks like it could be the culprit.

Is there anything I should be diagnosing before replacing the booster? Is there any way to check a junkyard booster (band name idea!) for this kind of problem?

Thanks!

[roguetoaster]

There are seals on both ends of the operating rod, the grommet that the vac line connects to, and the diaphragm (which can leak internally or along the entire crimped perimeter). Generally, boosters are fairly reliable, so you'd have good odds of a random used one working. As to testing, a smoke test or submerging it in water while applying low air pressure might be the easiest ways forward.

[blalor]

Are the seals on the operating rod replaceable? Given that I hear the noise in the cabin I'm inclined to think it's not the line or grommet.

[roguetoaster]

In a practical way, probably not, I'd assume you'd need to fully disassemble the booster to service any of it. While I bet you could do it, I'd worry about the diaphragm not going back in place while you try to put the halves back in place with no factory type jig or tool.

In a quick and dirty sort of way, you could try and swell the seals with some type of spray that you can get in an aerosol, but you'd really need to know seal material type first. It also wouldn't be likely to last.

[blalor]

Ok, thanks for the confirmation. I think the halves are crimped together in some fashion, so not easily disassemble-able. I hate to throw parts at a problem, but that's probably what I'll do here. Unfortunately it's somewhat involved to remove the booster (brake pedal, fluid reservoir, brake output cylinder, etc) and I want to avoid having things taken apart longer than necessary. I'll try to snag a booster from the junk yard this weekend, otherwise they're not too expensive on eBay.

[roguetoaster]

There are also new ones out there, in the range of $200-250 if you'd like to try that, and providing any are in stock right now.

[blalor]

They're between 250 and 1400 and not in stock that I can find anywhere. 😆😭