Vacuum brake booster ‘97 Lesabre

[used red MN]

Is everyone that has any knowledge about brake boosters agree that if I hear a noise out under the hood when pressing the brake pedal that there is a problem or malfunction of the booster? The noise is a whistle of sorts, it actually sounds like a very faint drum brake squeal but it does it when pressing the brakes with the car stopped. And no, it is not a noise coming from the ABS unit.

 

[Steve Metcalfe]

Is everyone that has any knowledge about brake boosters agree that if I hear a noise out under the hood when pressing the brake pedal that there is a problem or malfunction of the booster? The noise is a whistle of sorts, it actually sounds like a very faint drum brake squeal but it does it when pressing the brakes with the car stopped. And no, it is not a noise coming from the ABS unit.

Your diagnosis sounds right to me!!

 

[Eman85]

Maybe... Check the plastic check valve that pushes into the booster and the hose for any leaks. Usually the booster noise will be heard in the car if the diaphragm leaks. Is the booster working correctly. With the car off pump the brakes multiple times deleting the vacum in the booster. You should feel the pedal change as the vacum vents. If it didn't feel that way that could be booster going bad. after pumping keep your foot on the pedal and start the engine, you should feel the pedal drop as the booster recharges.

 

[used red MN]

Is the booster working correctly. With the car off pump the brakes multiple times deleting the vacum in the booster. You should feel the pedal change as the vacum vents. If it didn't feel that way that could be booster going bad. after pumping keep your foot on the pedal and start the engine, you should feel the pedal drop as the booster recharges.

Yes, it passes those two checks.
How about symptoms of master cylinder failure. For me it has always been 1) leaking at piston on back. 2) Pedal fade when brakes are held applied, in this case sometimes the fade will stop under heavy brake application but present at a more noticeable level during lighter brake application.

What other failures have called for a M/C replacement?
The new symptom that just came on this week seems to be inconsistent brake feel and application, more noticeable in a panic stop.
And I am no idiot, the fluid level is adequate.

 

[504]

Unplug the line at the booster and look for the grommet cracks and checks the hose for leaks at the fittings too.

 

[Nordic]

Quote:
Yes, it passes those two checks.
How about symptoms of master cylinder failure. For me it has always been 1) leaking at piston on back. 2) Pedal fade when brakes are held applied, in this case sometimes the fade will stop under heavy brake application but present at a more noticeable level during lighter brake application.

For me, these symptoms you are quoting have been the master cylinder beginning to fail.

 

[Eman85]

Test for MC is buy 2 brass plugs, remove lines, screw brass plugs into MC and step hard on pedal and hold it. If the pedal is rock hard and doesn't drop MC is good.
Now let's join the real world with a 30 yo car that has probably never had the brake fluid changed. Of course we're all in our easy chairs and ASSuming you have removed all of the wheels and carefully inspected the operation of each wheels brakes and made sure there are no leaks. Rear wheel cyls. are prone to leaking and seizing. If I wanted reliability I'd go on Rock auto and replace all of the hoses, MC and wheel cyl. and calipers. probably spend $200 in parts.

 

[used red MN]

Of course we're all in our easy chairs and ASSuming you have removed all of the wheels and carefully inspected the operation of each wheels brakes and made sure there are no leaks.

And I am no idiot

But maybe I am after all.
I’m guilty of not giving the rest of the story, guess I need some finger pointing. No I have not had all the wheels off and checked them over. But there are no visible leaks and the MC fluid level was maintained where it has been.
Prior to this winter it always stopped well. During the end of last year the ABS light started coming on intermittently. Then a line rusted through at the right rear. Replaced that, had difficulty bleeding but for shame on me I did not bleed it to the rear cylinder only to where the last connection was made in the line. There is a hose that attaches it to the rear control arm and then a short tube up into the wheel cylinder. I did eventually get it bled to the point I thought was satisfactory, however I don’t think the brakes were as good as they had been previously. I had two a Lesabre for a time the other was a ‘99 and this is a ‘97. The ‘99 never did stop as well as this ‘97, so I am just thinking this was the level of braking it was going to have. Also, it seems the more consistent temperatures have kept the ABS light off, if that has any correlation or not.
Something happened this last week that changed the pedal feel and the brake response on the first application. A second pedal pump does seem to improve braking.
I drove it around the neighborhood last Thursday evening checking out the brakes. That is when I noticed the noise described in the OP. In the past,I have heard this noise faintly occasionally after start up, but only occurs once and then all is good.

 

[used red MN]

Update, this last weekend I pulled the right rear where the brake line had rusted through this winter. I caressed open the bleeder and bled the wheel cylinder, I did get a couple of spirts of air. Took my angle grinder and removed the ridge on the brake drum. Removing the ridge allows you to adjust the brake tighter because it does have to contract the shoes enough to go inside that smaller diameter. Presto, seems the brakes are back to good as ever.
Lesson learned to always bleed all the way back to the cylinder not just where you broke the line you are replacing.