1994 Chevy Suburban Semi soft brake pedal, excessive travel

Tiny
BNKM1LL3R
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  • 1994 CHEVROLET SUBURBAN
Brakes problem
1994 Chevy Suburban V8 Four Wheel Drive Automatic 270k miles

The suburban's brakes began feeling inadequate (the rotors, pads and shoes are relatively new). I replaced the right front caliper and hose (was not fully releasing) cut the rotors, sanded the pads (getting rid of any glazing) and thoroughly bled the entire system (replaced fluid as well). The initial bleed was at the replaced caliper followed by the standard routine*.

The pedal still has excessive travel, a semi soft feel and aprox 70% braking ability.


Next I examined the entire system - no leaks and the rear brake area is dry. So I removed and replaced the master cylinder, which was thoroughly benched and re-bled the entire system once more. I don't believe the booster is at fault, I checked it as well.

The pedal is still semi soft with excessive travel and about 70% braking. There is no pulsing in the pedal (like ABS activation). As a matter of fact, I can't brake hard enough to activate the ABS. What is going on here?

PS. There are no lights lit on the dash and all of the
lights are functional.

* Bleeding was accomplished as always beginning at the right rear, left rear, front right, front left.
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Wednesday, June 25th, 2008 AT 3:54 PM

5 Replies

Tiny
RASMATAZ
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On some ABS-equipped vehicles, you may also have to cycle or reposition the ABS solenoids or valves with a scan tool. Many also have additional bleeder screws on the ABS unit for bleeding the system.

Try rebleeding it start with the M/C then the ABS unit if it has one-then the wheel sequences
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Wednesday, June 25th, 2008 AT 5:58 PM
Tiny
BNKM1LL3R
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I followed the instructions for bleeding the 4wal kelsey hayes system as specified on cardone. Com. Actuated the selenoids while holding the high pressure acumulator buttons and having the internal bleeds open 1/4 to 1/2 turn.

I've now bled the system to the tune of 1 gallon of fluid.

I bled with the method described on cardone, then bled them thoroughly twice using the "standard method".

The brakes still are soft and will not kick the abs function. I did notice that my front tires will lock on gravel (but only under extremely hard braking).

And. As before no lights - no codes. Please advise.
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Thursday, June 26th, 2008 AT 2:22 PM
Tiny
RASMATAZ
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The wheel cylinders are they okay not leaking and also the brake shoes are they adjusted properly.
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Thursday, June 26th, 2008 AT 2:32 PM
Tiny
BNKM1LL3R
  • MEMBER
Yes. Rear drums have a very slight drag (when brakes are not applied), the calipers are working correctly (wheel bearings are not wobbling) and the entire system is dry.

I took it to a dealer today. They bled the brakes using the special tools to depress the buttons on the high press. Accumulators and distr. Block while actuating the ABS unit. ! No air ! Unbelievable! The tech then asked if I had the rotors turned lately (and I did), he then asked if a "special fixture" was bolted to the rotor while it was turned (which wasn't). He said that composite rotors require this fixture and a different cutting bit. They recommended to change / cut the rotors and put new pads on it.

They told me several stories of composite rotors not being turned correctly and crappy pads which caused the driver to use excessive pedal force to stop the vehicle. I didn't know which pads were installed (installed by the previous owner), so I replaced the rotors and pads when I got home.

I installed pads which have very distinct burnishing procedures - so I'll know in the AM (after they've cooled) if it worked.

Is this BS or could there be some truth to all of this? And. How can you tell by looking at a rotor, if it is a composite rotor? I assume composite means composite alloy.

! Thank you for your continued diligence !
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Friday, June 27th, 2008 AT 11:04 PM
Tiny
RASMATAZ
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Here's some info on it

Composite brake rotors have been around since the early 1980s. When they were introduced, everyone was worried about soaring fuel prices (just like today), and auto makers were scrambling for ways to take weight out of their vehicles so they could meet government Corporate Average Fuel Economy (CAFE) standards. So one of the changes that was made was to replace heavy cast iron rotors with lightweight composite brake rotors.

COMPOSITE ROTORS SAVE WEIGHT

This type of rotor derived its name from the fact that it combined a stamped steel center hat with a cast iron rotor. Thus, it was a composite of two different materials. The new design proved to be about 20 percent lighter than a conventional one-piece cast rotor and saved up to a couple of pounds per rotor.

The composite design also allowed the rotor disc on some applications to be cast out of a special "dampened" gray cast iron for added noise suppression (dampened cost iron is not structurally suited for use in a one-piece cast rotor).

Some of these rotors also featured redesigned cooling fins for better heat management. Some were also directional for use on either the left or right side of the vehicle. Directional rotors are not interchangeable side-to-side because reversing the direction of rotation changes the cooling characteristics of the rotor.

COMPOSITE BRAKE ROTOR APPLICATIONS

Some of the earliest applications for composite rotors were the 1982 Lincoln Continental, 1984 Ford Mustang SVO, 1987 Ford Thunderbird Turbo Coupe, and 1988 Ford Taurus and General Motors front-wheel drive "W" body cars (Buick Regal, Oldsmobile Cutlass Supreme and Pontiac Grand Prix). Since then, the number of vehicle applications has continued to grow.

SERVICE PRECAUTIONS
As the vehicles with composite rotors accumulated mileage and came out of warranty, the aftermarket discovered that composite rotors required some special service procedures. Because the stamped steel center hat is not as thick nor or rigid as that in a one-piece casting, the center hat on a composite rotor must be fully supported with special adapters or oversized bell caps when the rotor is resurfaced on a brake lathe. The other alternative is to bolt the rotor to the hub (with the lug nuts reversed to provide better support and to prevent deforming the center hat) and to then resurface the rotor with an on-car lathe.

If a composite rotor is not supported properly, it can wobble and flex creating runout and surface finish problems. Both sides of the center hat must also be clean and rust-free for the rotor to turn true.

ROTOR RECALLS

Some of the early composite rotors experienced corrosion problems and were recalled for replacement. Ford switched some of its models back to conventional one-piece cast rotors for awhile, and issued a service bulletin (#91-8-9) saying it was okay to replace composite rotors on the 1986 to 91 Taurus and 1988-91 Continental with one-piece cast rotors (P/N F10Y-1125-8). The corrosion problem is mostly history now because the OEMs now apply a protective coating to the stamped steel center of the rotor to inhibit rust. Aftermarket composite rotors typically use stainless steel for the center section to prevent rust, and the circumference of the center section is also dovetailed (notched) for increased strength where it mates with the rotor.

Vibration problems were also blamed on the design of the composite rotor. But the underlaying cause more often than not turned out to be improperly torqued lug nuts. Any rotor can warp if the loading on the lug nuts is not even. That is why an impact wrench should never be used to tighten lug nuts, unless it is used with a torque-limiting socket. Otherwise, a properly calibrated torque wrench should be used to tighten the lug nuts in a star or cross-pattern sequence.

REPLACE ROTOR
When worn or damaged composite rotors need to be replaced, be careful not to intermix rotor types side-to-side. Rotors should always be the same type on both sides. Replacing a composite rotor on only one side with a cast rotor may create a brake pull. So do not replace a composite rotor on one side of a vehicle with a cast rotor unless the rotors on both sides are being replaced.

Cast replacement rotors for vehicles that were originally equipped with composite rotors are available from various aftermarket suppliers. But other aftermarket suppliers recommend against substituting one type of rotor for another because the cross-section of the center hat on a cast rotor is significantly thicker than the stamped steel center hat on a composite rotor.

The difference may range from 1/8 to 1/4 inch or more depending on the application. This may not sound like much, but it does reposition the wheel slightly further out on the hub. This adds positive steering offset and alters the scrub radius of the steering geometry. The amount of change is not great, but neither is the amount of scrub radius that is designed into many vehicles. Scrub radius affects steering feel, steering effort and steering feedback. It also plays a role in the way braking and engine torque affect steering.

The change created by replacing a composite rotor with a one-piece cast rotor may be enough to alter the scrub radius from negative (which is the case on many front-wheel drive cars) to zero or positive offset. This, in turn, may create a noticeable difference in the way the steering feels and reacts, especially on cars with rack & pinion steering that are especially sensitive to steering feedback.

There is also a concern that substituting a thicker cast rotor reduces the overall length of the lug studs, which reduces the number of threads available for the lug nuts to retain the wheels (especially on thicker alloy wheels).

Suppliers who subscribe to the "replace same with same" philosophy say you are apt to encounter far fewer problems when you install replacement parts that are the same design and function as the original. Those who do not subscribe to this philosophy say there is often room for improvement over the OEM design
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Saturday, June 28th, 2008 AT 2:02 PM

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