Brake Fade and hum

Ok,

The mighty Rivs brake issue has either gotten worse or we are just paying a lot of attention to it.

When I apply the brake at any amount of force from about 25+ MPH, the brakes feel nice and strong. But as I slow the pedal sinks in. Sometimes it goes to about the floor.

It has brand new pads and rotors all around...

Also the back breaks while stopping/slowing on the slow side (sort of a drag) they will make a "hum" noise.

Thoughts?

Power booster going bad? Master cylinder? Something else?

I checked for vacuum leak on the power booster hose. Its solid/tight.

When my master cylinder went bad the pedal got softer and softer until i had my foot through the floorboard. I would try to get it fixed quickly especially if it is the master cylinder since my brakes completely went out on the way to the shop.

Sounds like your master cylinder is leaking down to me.

Agreed. Change the master cylinder. They aren't too hard or expensive. The piston seal breaks down and leaks on the inside, causing loss of pressure.

As for the hum sound, it's probably a separate issue. Do you notice any vibration? Are the rotors drilled or slotted?

Ok, will get the Master Cylinder going ASAP. Just wanted a general consensus as money is tight at the minute.

The brake hum: The rotors are just plane from Autozone. Same with the pads.

sure its the rear brakes making the noise and not the tires? do you notice any unusual wear on the rear tread? like the edge is feathered or cupped?

Its for sure the brakes in the rear.

The car is smooth and quite, like a slow moving missile With the exception of the brake hum/vibration.

I will see if I can get a sound clip of it later tonight or tomorrow.

I'd check that the pads are seated correctly and wearing evenly.

Also, while your in there check to see if the rear hubs have too much play.
Bearings could be source of humming while under brake load.

Ok, back the the master cylinder issue.

With the car off I pumped the brakes, then held. The peddle felt stiff even after a minute or more. I dont think it went down any...

So it makes me think maybe the master cylinder is ok?

If thats true then what? Power booster? Do those go bad? I see theya re available...

Try pressing really hard.

With the car off, I don't think much pressure is applied to the master cylinder. The pedal wouldn't sink, just like if you were rolling with engine off, the car won't stop.

Imo, there isn't any other reason for the pedal to sink other than a leak. Check all of your lines and calipers for leaks. Check fluid level, too.

I never really knew how power boosters worked so I read up on howstuffworks.com

I agree now its not the power booster. Argh dont really want to replace that master cylinder but theres no choice in it.

mayht be even air in lines. but..not very sure..cos it came after new bads and rootors?...but yeh..could be leaking tho somewhere..
mine sinks do the floor tho...but...i will fix it after i get new rootors and bads for mine.

Ryan from Ohio wrote:

I never really knew how power boosters worked so I read up on howstuffworks.com

I agree now its not the power booster. Argh dont really want to replace that master cylinder but theres no choice in it.

Try and bench bleed the master cylinder before installing it.
Directions on how to do that (if you don't already know) should be with the new part.
It will save you a lot of time and aggravation later, when trying to bleed the brake system, less air to deal with.

yea, and the air wont get into the abs modulator
(abs randomly kicks in while drivng to recalibrate whe it sees the air) not fun, but at lease i finally got all the air worked out.

I think Im going to hit up a yard and pull a cylinder. Its $25 that way, new one is $60 @ Autozone. I would prefer new but what the heck, go used with this one.

The issue has been present even with the old brakes.

i would jus go and buy a new one. you never know with a junkyard part. the primary and/or secondary seals on the pistons may have become brittle from prolonged use.

plus you dont know how long the car has sat at the yard. most of the cars i see at the bone yard have major water damage from being half torn apart. water and brake fluid should never meet. even with a bench bleed i dont know if you could completely clean out the old unit.
not sure what year riv you have ryan (turn your sig on!), but rockauto has MC's for as little as $40

Don't try and save money on this one. Not worth the hassle if you get a bad one at the bone yard.
Always use new parts when you can for your brake system.
After all, they are what stop your car when you want them to.

Well I decided to check the yard out anyhow. I was searching for some other parts for my Alero.

There it was. Brand spanking new master cylinder for $20.

Theres not a drop of ruse. The reservoir is crisp white yet. lol

So yup I bought it.

Looks like someone did some work to the car before it had an accident... lol Good for me

Well the new master cylinder seemed to fix the fade issue.

But the hum is still present a little bit. Seems to me its the passenger side. While bleeding the brakes it appears that its loose on the top pin. I think it might be wore.

Not sure. Might just replace in a few weeks anyhow out of caution.

Ryan from Ohio wrote:

But the hum is still present a little bit. Seems to me its the passenger side. While bleeding the brakes it appears that its loose on the top pin. I think it might be worn.

Not sure. Might just replace in a few weeks anyhow out of caution.

When you replaced the rear brakes did you replace the sliders (abutment shims) and bolts. I know that the last time I replaced my rear brakes I had to replace all the misc. pieces due to heavy corrosion. They were bad enough that a good wire brushing did not clean them up enough for proper operation.
You also need to use Synthetic Caliper Grease on the sliders and bolts so everything will move back and forth with the operation of the brakes, otherwise you can get get squealing, humming, etc.
The hardware kit and grease are available at most Auto parts stores.
If you didn't clean and lube the front calipers, I would do those also, and inspect the condition of the sliders and replace if necessary. They are relatively inexpensive.
I take mine apart every year for cleaning and lubing because the winter salt and sand takes its toll on the brakes. Just good preventive maintenance.

^^^agree with Rick, good slides are important. I just purchased a new set of Raybestos slider hardware for all 4 wheels when I did my brakes. I think the entire set was only $20 or so, maybe a little more? They are so far holding up pretty well after a year of operation. Cleaned up shiny and new with a wire brush last time I had them off.

But if the hum is that loud, I think there's something else that really needs attention. Inspect the rear pads and discs thoroughly when you have a chance. I don't think just having rusty slider hardware would typically cause a major problem like this.

AA wrote:

But if the hum is that loud, I think there's something else that really needs attention. Inspect the rear pads and discs thoroughly when you have a chance. I don't think just having rusty slider hardware would typically cause a major problem like this.

I agree that there could be something else wrong, but a good starting point for proper diagnosis is to take things apart, inspect, replace sliders and hardware if necessary and lube.
Then if the noise persists you can start to look at possible hub bearing for noise.
In other words do the easy and least expensive stuff first and move on from there.

Just before this I did a full pad/rotor replacement.

The brake with the hum, well the top slide is loose there. The rubber cover is torn.

The slides are good. I have caliper grease and use it heavily

I think the caliper is just worn. Only thing I can think of.