Brake booster question

getdealtwith · Apr 15, 2022

My '04 Navigator's idle has been dropping / stumbling at stop lights a couple of times. This doesn't happen frequently or consistently though.

What does happen ever since my mechanic replaced the master cylinder and brake reservoir because of a leak is that after cold starting and backing the truck out of the downhill driveway (applying the brakes) is a weird pop / release noise from where the diaphragm is located near the brake pedal.

I'm not experiencing any other symptoms of a failing brake booster, no hissing sounds and the vacuum line to the intake manifold is new.

Should I replace the booster? It definitely has some surface corrosion from the brake fluid but I've seen that before without any problems. (Pics attached)

whtbronco · Apr 18, 2022

Have you looked up under the dash to see if the issue is inside?

I can't say I have ever heard a popping noise from a faulty brake booster, though I'm sure it's possible. It's usually a vacuum leak. I sure have heard popping noises from faulty switches or the accordion style boot that covers the pushrod from the brake pedal to the booster.

Hamfisted · Apr 19, 2022

Where was the old master cylinder leaking ? If it was leaking INTO the brake booster, then yes, it can damage the vacuum booster and the booster may need to be replaced. Like Bronco said, look up under the dash with a bright flashlight, and see if you see any evidence of brake fluid leaking in to the pedal area or getting into the switches that are mounted on the pedal.

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getdealtwith · Apr 24, 2022

I'm not seeing any signs of leaks. The noise sound a bit like it could come from the front left strut, too and now that I think about it, it started after I had the new struts installed. I'll need to have the shop look at it again.

whtbronco · May 2, 2022

Any luck identifying the noise? It could be the strut bellow. I've had several make popping noises as they move around. Honestly I wouldn't run bellows on the street. Short of keeping mud and rocks off the rod I think they are more trouble than they are worth.

getdealtwith · Jul 14, 2022

whtbronco said:
Any luck identifying the noise? It could be the strut bellow. I've had several make popping noises as they move around. Honestly I wouldn't run bellows on the street. Short of keeping mud and rocks off the rod I think they are more trouble than they are worth.

After listening more closely under the dash when first backing out of the driveway, it's definitely coming from the brake booster so I'll be replacing that. My strut conversion kit didn't come with bellows.

I'm pretty sure I've identified the source of the idle stumbling though: I started getting a P2195 code and thanks to Mode 6 data, was able to see that under load, one of the relatively new O2 sensors was telling the ECM to add fuel at 50+ % (short term fuel trim). It was the upstream, passenger one (bank 1, sensor 1) that is very hard to get to between tranny, exhaust and chassis and I found it slightly loose when I replaced it yesterday. No stumbling or codes since, after taking it on a longer test drive.

whtbronco · Jul 14, 2022

Good news. Always nice to see a follow up with resolution.

getdealtwith · Jul 14, 2022

I also need to correct my statement about the bellows. The strutmasters kit does have them. Just saw that when I was under there spraying down my bushings and ball joints. It was definitely a valid suggestion given the timing of the replacement and noise fist appearing, plus my suspension gets awkwardly compressed on the sloping driveway, when parked overnight. Appreciate the support here!

Brake booster question

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