I have heard of people needing new calipers shortly after a brake job. As the pads & rotors wear, the effective range of motion changed for the calipers. Gaining all the extra brake material, thus recessing the caliper piston, can put the piston in a weak spot. All brake functions and sensors will operate normally, but the actual force applied to the rotor decreases.

Another cause is a rough caliper track. Many people forget to remove all rust & burrs and to lightly grease the caliper track on the brake bracket. There is often enough debris/rust/etc to distort and reduce pressure the calipers apply to the rotors.

Although the odds of all four having the same symptoms are incredibly low, I have seen it.

 

thanks for the reply......i will contemplate this. thanks for the suggestion. ps...caliper track is clean and lightly lubbed.

 

UPDATE....after a brief road test, after sanding off the glaze on my rotors and pads, my braking has not improved at all. i contacted a shop and they seem to think that i was given the wrong pads (cheap ones) for my 88.

can anyone tell me what the STOCK oem replacement pad was for my year (88)? i don't need to know what everyone runs in their cars, just what the oem was as far as composition (ceramic, etc)....thanks!

 

The OEM pads were a semi-metallic compound but not nearly as much metallic composition as aftermarket high performance pads.

If your current pads are the ceramic type, you would probably see reduced braking performance when they are cold.

At this point, I would recommend going to a high-performance street pad to imporve braking performace given the fact that you have obviously checked everything else in the service brake system. I would suggest the Hawk HPS pad or the Performance Friction PFC Z-Rated pads or even the EBC GreenStuff pad.

I have use both the Hawk and EBC pads for street use, autocross, and even a few track days and found them to be head and shoulders above stock OEM pads in terms of braking power when hot and especially when cold. You will get more dust on the wheels (EBC pads dust very little ) but it simply washes off with any of the available wheel cleaners.

 

thanks for the reply....the parts house sold me "wearever silver"
#'s mkd-413 +414's semi metalic pads.

i have priced out a few of the pads and looks like i will have to get a mortgage on my house to afford them!...if an aftermarket pad states that it is "oem" or "oem performance", should not that pad perform at least at a "base minimum" to what the factory put on when the car rolled out the door?...should they not have generally the same braking power?

i found a mic (micrometer) that i didn't even know i had. however, i can't figure out how to read the darn thing! ugh!..the rotors are plenty darn thick and i finally found a place that could turn them. however, to save me a lot of time, i was going to measure the thickness of the rotors before i took the off the car.

 

UPDATE;.....

after having my rotors turned, i finally road tested my brakes today...NO CHANGE!!! *$@%^%$&^ . the only two things that have not been changed in the braking system is the abs pump and the vacuum booster.

the abs initializes when i back the car out as i can hear it. i have applied vacuum to the booster and check valve and they hold vacuum.

***could it be that the booster will hold vacuum until the pedal is depressed and that is where a leak may be? i know that when you depress the pedal, outside air is supposed to rush in (so-to-speak) and give you "the power".

however, could mine be leaking when it is depressed? i can "stand" on my pedal and the brakes don't change much if any. if i shut the car off, i can pump the pedal 2-3 times and expel the vacuum in the booster and the pedal will become firm. any help will be appreciated as i am down to no more solutions.

 

Hey Mail Man, do you have the gm manuals? I suspect from other conversations that you have, and have probably checked the diagnostic recommendations. But, just in case you haven't and don't, drop me a line and I will search mine at home. Just reading the posts I don't know what else to suggest, all of my experiences have been cured by the previous suggestions.

 

thanks for the reply...yes, i have the helms manual and have looked at it multiple times and have not found an answer. the only things that i have not replaced are all the calipers, abs pump, and brake booster. i can't see me using the "shotgun" approach to find the trouble.

abs energizes when backing and no codes, flushed system, bled system 5x's, rotors turned, stainless steel brake lines at wheels, m/c replaced..ugh! i wonder if it is possible for the booster to have a leak when the pedal is pushed. i know that air rushes into the booster to give "power" but, could it leak while pushing the pedal?

i have applied a vacuum to the booster and valve with a "miti-vac" vacuum tool and it appears to be holding vacuum like the spare i have in my garage. if my car is running and i shut it off, i can depress the brake pedal 2-3 times to expel the vacuum from the booster and the pedal will become very firm. i hear no "wooshig" noise while pressing the brake which would indicate a leak....

just a wild thought; is there a decent way to check the calipers to see if they are actually expanding against the brake pads?..maybe they could be hanging up somehow?..i have lubed the "pins" and the rear "slides" for the calipers and everything appears to move freely......again, there are no leaks anywhere.

 

Tough questions. The thing that is odd is that for your system to be as weak in breaking as it is would mean that all four corners would have to be defective, and the odds of that happening at the same time are astronomical. Even to have a pair on the same axle fail would be unusual, which tends to point to some other unit within the system that would effect all four braking points. You can probably concentrate your attention on the fronts since the majority of braking occurs in the front. It would seem the decision now is whether to take it to a pro (such as a brake shop who can test the booster). You could start with replacing the calipers, but that gets rather expensive, especially when you have to do it in pairs. Wish I could help more...

 

What are the specific performance indicators you see that are telling you there is a problem? After reading the posts here, it sound like you have checked or replaced/repaired the components that would point to poor braking performance but you still say there is some issue.

Have you tried to measure stopping distances on a road? Full brtaking power stops from 35 to 0 should only take 30-40 feet. From 60 to 0 should be around 110-125 feet for OEM brake pads. The car should stop in a straight line without having to do any steering correction. With the ABS working properly, there should be no wheel lockup other than what the ABS controls.

Maybe if you could elaborate on some specific things like posting the results of stopping distances, it may help if we knew what the car is doing when the brakes are applied.

When I first got my 92, I questioned braking performance compared to the 78 I had owned. As it turned out the 92 was stopping smoother that the non-ABS 78. Braking distances for the speed ranges I mentioned bove were shorter with the 92 compared to the 78. It just felt smoother.

I got to the point with my 78 where I could modulate the pedal to just before the wheels would lock up which is about the best braking performace, but the 92 with the ABS must made it easier to use the full potential of the bigger and better brakes and much better tires.

When I installed a set of Hawk HPS pads in the 92, I immeditely noticed a big difference in braking performance in terms of felt stopping power. This was due to a much improved cold bite with the Hawk pads. "Cold bite" with performance pads means that braking power is much better compared to stock pads when the brakes are cold to the touch. As the Hawk pads got hot, the brakes got better.

 

thanks for the reply..i agree this this is very strange to say the least! i agree that all 4 corners couldn't be bad and so on.

the booster checks fine with alternative source of vacuum applied to it. unless i am doing something wrong in testing it (with and without the check valve) , i find no fault in it. however, i am running out of things to replace...looks like the booster is next. i have another good known booster in the garage and both have been checked in the same manner (exterior source) and with the same results (so-to-speak)..obviously 1 not being on the car!. i need to determine how i can check the calipers while on the car. (i can remove them from the rotors but, depressing the brake may "shoot" the piston out!).

 

thanks again for the reply.....i never measured the distances but, i tell ya, the braking is bad and that is compared to ANYTHING!.

at 30 mph+/- , i can stand on the brakes and the nose will dive a bit but, the car just seems to glid to a stop. if i was on interstate driving, i would most certainly be a fatality. i have no brake pulling to either side whatsoever.

i can probably get the pads hot enough to "smell" but, even at that, fred flintstone and his clubby feet have me beat hands down.....

 

***took a chance and went up to the store. i was doing approx 30mph and stood on my brakes. the car should have abruptly stopped, but ,it didn't as i think it should have.

then, approx the same speed again (+/-), while standing on the brakes, pressed the accelerator pedal down just a bit, nothing major at all and was able to more the car, although slowly somewhat.....i hear (i think) the brakes trying to grab (if ya know what sound that makes especially after getting the rotors wet!) but, cannot determine which end or ever "how many".....do ya think if i somehow disconnected the abs that it would prove something?...i know that the abs CANNOT increase line pressure.......thanks again all!!!

 

hmmmm... I wonder if you could be on to something. Within the abs system there has to be a valving system to control the brake application. What if there is a valve that isn't functioning correctly, or is partially functional due to crud. You said you have flushed the rest of the system, is there a flush that would clean the main control unit? I wonder if that area gets flushed during the course of a normal flush.

 

thanks for the reply!.....interesting eh'? i would think that if anything were screwed up with or w/in the abs pump or associated hardware, it would throw a code at me, but, it hasn't. yes, i have flushed the system many times and am thinking of investing in a 55 gallon drum of dot 3 brake fluid!..grin...

i imagine that since the fluid goes into the abs that it also must flow out of it, and thus - break force. the helms says that if the abs takes a dump, then the brakes return to standard power brakes with no abs function.

i have read some posts of where some have found a bleeder port on the abs and have bled it from there as well as some posts stating that there is a machine that is nesessary for completing this. i have always bled brakes "the old fashioned way" and never have had any problems.

 

The ABS pump itself does not usually require bleeding and fluid does not really flow thru the pump from the master cylinder out to the calipers.

Outside of replacing the ABS unit itself, most places do not bleed the ABS unit. In the Vette, you must use a GM Tech-1 Scan Tool with a brake module to bleed the ABS pump. The Tech-1 is needed to operarte the pump in order to flush or load the pump.

While the pump does run at start-up (when the car hits 3-4MPH) a small amount of fluid passes thru it but it simply pulls fluid from the master cylinder and returns it through another line back to the M/C. There is no flow of fluid to the individual wheel cylinders. If the lines to the calipers are full and there are no air pockets, there is no other place for the fluid to go.

The valving system that thebeerman referred to is the proportioning valve and is located in the master cylinder. It's a spring that is designed to vary the brake fluid pressure between the front and rear brakes to insure that the rear brakes don't see too much pressure or that the fronts see too little. These typically don't fail and if you have replaced the master cylinder, it should not be an issue.

BTW, did you bench-bleed the master cylinder when you put it on the car??

 

thanks for the reply....yes, i did bleed and replace the m/c which is the first thing i did. i am aware that the proportioning valve is located in the m/c and for the most part, is non servicable outside of just changing the m/c. i was not aware that little or no fluid flows through the abs pump.

that raises a question; if no fluid flows through the abs, how then is the pressure "modulated" to each individual wheel in the event of a "slide"? i realize that the abs gets some of it's info from the wheel speed sensors but, it would have to pulsate-modulate the fluid to or from the "offending" wheel caliper to obtain the desired results it was designed to do.

i knew that there was some sort of device nesesary for servicing the abs but could not remember the name of it. also, i do get that self test "groan". thanks again for the reply.

 

The 88 ABS system does not set codes. The system used from 86 - 89 was essentially the same and it does not retain or set codes like the later ABS systems. Kent-Moore is the specialty tool supplier for GM and they made a special ABS Test Tool J35890 and Pinout Box J35592 just for testing the 86-89 ABS control module and system components. I am unaware of any other tool including the Tech-1 that can test the early ABS systems.

If the control module sees a voltage probelm, or other operating problem it will turn on the ABS Anti-Lock light and disable further ABS operation. That is all the indication that it will give you. The modulator valve cannot increase brake fluid pressure, but it can act to release pressure. It is possible that the modulator valve is mal-funtioning and maybe causing your problem. You can disable the ABS controller by unlatching the controller input harness connector from the controller behind the driver's seat. Sections 5 and 8A-44 of the 88 Factory Shop Manual deal specifically with the methods for testing and replacing the ABS system components.

 

thanks for the reply..i AM aware that the abs CANNOT increase brake pressure however, i did think that the abs would throw a code if something were wrong. i guess i thought that the abs light was a code of sort which i still believe it is although different from the typical codes. is there not a fuse for the abs?

 

There is not a separate ABS fuse for the early C4s. It relies on the Guages Fuse and Brake Fuse to the Module Relay, which supplies battery power to the Control Module. Also involved are the Stop/Hazard fuse via the Brake switch, and the INJ1 Fuse, plus the Fusible Link.

The Control Module does not set or retain a code. It is an all-or-nothing type of system. The Anti-Lock light is turned on and ABS functionality is disabled for any of several reasons. It is either functioning or it is not. Using the ABS Test Tool J35890 the tecnician can cycle the ABS system through certain tests and define the area of the problem that caused the Control Module to set the light. I bought one of these used from a retiring Corvette Mechanic via eBay a few years ago.