Mushy Brake Pedal

[07sierra8.1]

Hey guys...Got an 07 Sierra 2500HD Classic body 8.1L....disc brakes front and rear.

 

Since I got the truck in 2010, it had a real low kinda mushy brake pedal. If you get a load on the truck or have to panic stop, it makes you feel as if it isnt going to stop. I have tried bleeding them several times, no change at all. Put brand new loaded brake calipers & pads all the way around with new delco rotors and all new shoes for parking brake. It got alittle better but still not really the way you want your brake pedal to feel.

 

I didnt really notice how bad my pedal felt until I got my 86 Chevy K20 back on the road and its like night and day difference betweeen how a 30 year old truck can practically stop on a dime, and my 8 year old truck feels as if it has practiacally no brakes.

 

Not sure what to do next? Master cylinder?

 

 

[Jsdirt]

Either the master, or the brake hoses have degraded and are swelling when under pressure. Braided steel hose is a nice upgrade even if they're not - improves pedal feel noticeably.

[Doug_Scott]

Were the brakes bled properly? There is a specific procedure for ABS brakes to follow.

 

If it was mine, I would first take a pair of vice grips and block each flex hose off one at a time and see if the brake pedal feel changed. To do this correctly, you need to first block the hoses as close to the chassis as possible, then as close to the caliper as possible. Doing it this way will test both the caliper for air, and the flex lines for swelling on application. If none of these tests point out where the air is, chances are it is in the ABS section.

 

A master cylinder will rarely cause a low pedal on its own. A master that is starting to fail will generally show a low pedal under light application, but under hard application the pedal will not be as low due to the design of the cups inside the master. They will expand outwards to seal tighter to the bore when fluid pressure is high.

[newdude]

Tear apart the brakes front and rear. Clean and lubricate the pins and slide clips with synthetic brake grease. I'd change the fluid and bleed the system as well. GMT-800 HD's are known for crappy pedals.

[07sierra8.1]

Were the brakes bled properly? There is a specific procedure for ABS brakes to follow.

 

If it was mine, I would first take a pair of vice grips and block each flex hose off one at a time and see if the brake pedal feel changed. To do this correctly, you need to first block the hoses as close to the chassis as possible, then as close to the caliper as possible. Doing it this way will test both the caliper for air, and the flex lines for swelling on application. If none of these tests point out where the air is, chances are it is in the ABS section.

 

A master cylinder will rarely cause a low pedal on its own. A master that is starting to fail will generally show a low pedal under light application, but under hard application the pedal will not be as low due to the design of the cups inside the master. They will expand outwards to seal tighter to the bore when fluid pressure is high.

 

 

Whats the proper way to bleed ABS brakes? I always just did this...Started with Right Rear, Left Rear, Right Front, Left front. Never seemed to have an issue with any of my other ABS vehicles.

 

 

Tear apart the brakes front and rear. Clean and lubricate the pins and slide clips with synthetic brake grease. I'd change the fluid and bleed the system as well. GMT-800 HD's are known for crappy pedals.

 

Everything is brand new, I got all new LOADED calipers and rotors.

[Jsdirt]

My truck stopped like this when I used Wagner Powerstop pads and shoes (rear drum) on my Silverado. Same pads and shoes in my '86 Grand Marquis will just about put you through the windshield ... but on the Silverado, downright scary. Could tell on the first test drive. Figured they needed to break in. I was wrong - ended up throwing them in the trash just 8k miles later. They were terrible!

 

I ended up throwing EBC yellows on the front, and since I couldn't find EBC shoes (or any other brand I felt confident in), I went with the overpriced OEM shoes for the rears. Now it stops just as good as it did new.

[Doug_Scott]

 

 

Whats the proper way to bleed ABS brakes? I always just did this...Started with Right Rear, Left Rear, Right Front, Left front. Never seemed to have an issue with any of my other ABS vehicles.

 

 

 

Everything is brand new, I got all new LOADED calipers and rotors.

 

No idea, I moved out of the profession before ABS became the norm. I just know that there is a procedure for bleeding ABS if you want to remove old fluid. The vehicles you never had a problem with before did not have a low pedal to begin with did they? Your 07 was 3 years old when you got it, with unknown brake history. Not the same thing as your other cars. Someone here will be able to point you in the right direction for the procedure. Would not surprise me if it needs a Tech II tool.

 

My truck stopped like this when I used Wagner Powerstop pads and shoes (rear drum) on my Silverado. Same pads and shoes in my '86 Grand Marquis will just about put you through the windshield ... but on the Silverado, downright scary. Could tell on the first test drive. Figured they needed to break in. I was wrong - ended up throwing them in the trash just 8k miles later. They were terrible!

 

I ended up throwing EBC yellows on the front, and since I couldn't find EBC shoes (or any other brand I felt confident in), I went with the overpriced OEM shoes for the rears. Now it stops just as good as it did new.

 

When semi-metallic pads first came out this was an issue. Put a new set of pads on a car, machine rotors, then have to convince customer to wait 1k miles for brake pedal to become firm again, or put a set of organic pads on it every 5k. One of the joys of manufacturers downsizing brakes to save weight. Organic pads would create a scary fade issue in heavy highway traffic condition when installed on cars that came with semi-metallic pads.

[Jsdirt]

Oh great ... so I guess flushing the old fluid out of the Silverado is a bad idea. I guess I'll wait until the calipers & wheel cylinders seize to deal with that. When did all this crap change? I just did lines on a '00 Silverado, and was able to bleed all 4 solo with the Mityvac. Not many vehicles will have a hard pedal just using the Mityvac, without a second body to pump the brake a couple times per wheel.

 

 

That's what I thought happened - got organic rather than semi-metallic.

 

When I had the trailer hooked up, I almost couldn't stop! Thank God the trailer has 4 brakes of it's own ...

[Doug_Scott]

Oh great ... so I guess flushing the old fluid out of the Silverado is a bad idea. I guess I'll wait until the calipers & wheel cylinders seize to deal with that. When did all this crap change? I just did lines on a '00 Silverado, and was able to bleed all 4 solo with the Mityvac. Not many vehicles will have a hard pedal just using the Mityvac, without a second body to pump the brake a couple times per wheel.

 

 

That's what I thought happened - got organic rather than semi-metallic.

 

When I had the trailer hooked up, I almost couldn't stop! Thank God the trailer has 4 brakes of it's own ...

 

May not have been clear in my post. Should have added "if there is air in ABS section". If your pedal is good before repairs are done, then you can likely avoid having to bleed the abs area specifically. For example, if you are just changing hard parts after the abs portion, then no air should be getting into the abs section. If you change the master, and do it properly(bench bleed before install) you should also be able to avoid getting air into the abs section.

But, if the pedal is crap to begin with, you have no way of knowing if there is air in the abs section. Bleeding the conventional way is not a fool proof way of bleeding the abs section.

 

What you likely got when you couldn't stop with the trailer was brake fade. Contrary to popular usage of the term "brake fade", it is not that the pedal fades away, it is almost always a high, hard pedal that when you press on it is like pressing on a brick. Hardly moves, and does nearly nothing in the area of braking. Almost like when a booster stops boosting. Get the brakes any hotter and they will smoke, and if hot enough, it will boil brake fluid, giving you no brake pedal due to "air" in system. If fluid is old, it will boil the water out of the fluid first, and you will skip right past the fade stage, going directly to no brake pedal.

[Jsdirt]

Mine wasn't fade - was just crappy friction material. As soon as I switched to EBC up front and OEM rears, it was back to normal.

 

Gotcha on the ABS. Makes sense, because I've done quite a lot of vehicles with ABS and I've never needed a scan tool, nor had any other issues. A simple flush won't introduce air (if done correctly, of course).

 

I have to do the coolant, brake fluid and engine oil at some point. Never seem to have the time. When I finally get time, we'll get 4' of snow ... I just know it.