Noise from supplemental brake booster

hppants · September 17, 2014

Greetings

I'm the original owner of a 2003 1500 2wd pickup with the 4.8L engine. I'm hearing a noise from what sounds like a servo motor or similar stemming from a part that I have learned is the supplemental brake booster. This noise comes and goes intermittently. When I am accelerating and I hear the noise, if I release the accelerator (presumably increasing manifold and booster vacuum), then the noise will stop

I'm wondering if the booster is bad or the supplemental booster is bad? I get no "booster dashboard messages". I haven't checked the vehicle for any computer codes

I could buy any truck I want. But I really like this one and intend to keep it a while. The best way to do that is to maintain it properly

Any guidance would be appreciated. Thanks

hppants · September 17, 2014

I did an extensive search on this forum and found nothing. What I'm experiencing must be unusual. The noise I'm hearing from the Supplemental Brake Booster sound pretty normal - meaning that the noise doesn't sound like the motor is failing, in a bind, or otherwise fixing to take a dump. Sounds like a vacuum motor or servo motor working normally. Of course, it's not designed to run continually for even cycle on and off for extended periods of time. I need to get to the bottom of this. I learned that this device is used to supplement the regular brake booster. My truck has 4-wheel disk brakes (LARGE caliper piston surface area) and I suppose the concern is that if my vacuum brake booster fails, it's going to take a strong leg to stop the truck. Apparently, the supplemental booster (SB) kicks in when the vacuum in the booster falls below 6 inches of Mercury, and turns off when it reaches 9 inches.

When I accelerate, I hear the SB start. If I let off the gas, it stops immediately. Then it will start again usually under acceleration. This is telling me that when engine vacuum is lowered (under acceleration), the SB starts as expected. When I let up on the accelerator, engine vacuum rises sharply, and the SB stops, presumably because the regular vacuum booster has been replenished.

The SB is mounted where a check valve would go for a traditional vacuum booster.

What I think this is telling me is that I've got a pin hole in my diaphragm for my vacuum booster. The SB "supplements" the loss of vacuum in the booster under appropriate conditions.

Another theory is that my check valve is not working - not sitting on its seat, causing me to lose vacuum in the system. Again, the SB supplements this loss according to its setpoints. What I don't know is where is the check valve? If it's located within the SB? If so, I need to change the SB. If it's located within the vacuum booster, I think I can fix my problem by just changing the booster.

I'd rather not throw darts at this, but if all else fails, I guess that's an option too.

If I had a vacuum gage that I could mount in line, and see it while I roll down the street (plumbed outside of the hood), I might be able to tell more.

Edited September 17, 2014 by hppants