C4 brakes - How do I do this?

[Alfonso Gomez]

Yesterday I took my 87 Coupe to have my front wheels re-balanced at the local Western Auto. The mechanic there told me the left side pads (driver side) were practically new but the other side were worn down almost to the metal. When I got home, I took out the right side wheel and the caliper. I removed the worn pads and compressed the caliper piston. It moved easily enough with no trouble whatsoever. Then I installed new pads in that caliper and reinstalled it. Also I reinstalled the wheel. Then I started the engine and pressed the brake pedal 3 times. When I tried to turn the wheel, it did not move. After a few more tries it started turning easily. So I took the car for a drive around the block and did not exceed 30 mph. When I parked, I noticed that wheel very hot and a smell of hot brakes. I lifted the car and that wheel did not turn easily. Recently, both calipers were rebuilt by a reputable mechanic and new OEM pads installed. Also, a new master cylinder was installed. Both calipers were bled, so no air should be in the system. Both front brake hoses are new and were installed when the calipers were rebuilt. How do I test to find out if the calipers are bad or if the trouble is in the brake lines?

 

[Vettelt193]

if you are going straight, and hit the brakes, does the car pull to one side?

 

[Alfonso Gomez]

if you are going straight, and hit the brakes, does the car pull to one side?

Yes, to the left momentarily, then straightens out.

 

[Vettelt193]

well, that just about has me stumped... if your right caliper was doing all the work, like it seems since the pad was worn out on that side, it should pull to the right, not the left....

One note, pads should really be changed in sets, both sides.... did it pull to the left before you changed the pad, after, or both?

It is also very easy to bleed the lines, you may want to do it just to be safe... it really can't hurt anything, and brake fluid is so cheap you have hardly anything to lose.

 

[tkrphxr]

I ran ino a similar problem. When I hit the brake the car would pull left but when I started to pull away it would pull right. The problem was the right front flex hose. The hose had a resriction in it and would not allow full press to the right caliper but would also hold a resdual pressure when the brake was released. A quick check is to get the wheel to drag and then open the bleeder. You will get a little spurt of fluid and the wheel will free up if it is a restriction in the line. Hope this helps and good luck. Charlie

 

[tkrphxr]

Sorry I forgot to add that this will also cause excessive heat build up and pad wear on the right side.

 

[Skant]

Perhaps the piston is seizing up and not releasing all the way. If the rebuild of the caliper didn't go quite right, this could happen. It might be holding it down a bit while you're driving with the brake pedal up.

That would certainly explain why the pad on that side was completely warn down while the other side was still new'ish. And why a ride around the block at 30 mph would heat it up.

It wouldn't have to be cinching it very hard at all. You may not be able to feel it while you're driving.

Perhaps you should lift up the front of the car and see how the two front wheels roll by hand. In particular, see if both wheels free all the way immediately after you let the brake off.

If the piston can't move freely, it may make it slow or impossible to apply and release. Which could explain the momentary pull to one side.

Since you had to pump the brake a few times to get the brake to release initially, it sure sounds like a sticking piston to me.

- Skant

 

[vettepilot]

Your caliper assembly should slide easily from inboard to outboard. You can check for this when the piston is fully retracted, the entire caliper should be able to slide on the threaded pins that go through the steel bushings, isolated by the rubber bushings. If the pins are not clean and free to allow the steel bushings to slide, the caliper, when pushed out by the master cylinder, may "lock" or "freeze"in place, thereby causing the inboard pad to continue making hard contact with the disc.
The caliper housing also rides against the caliper bracket that is part of the spindle assembly. The areas where they make contact should also be free of rust or corrosion, and a little water resistant lube there won't hurt either.
Just remember when applying any lubricant near the brake components you should do so sparingly, you don't want it getting on either the pads or the rotor surfaces.
Most people check to make certain the piston is not frozen in the caliper bore, but may not check for corrosion or other foreign matter on the pins. If they have a rust or scale build up, burnish them lightly on a brass or steel wire wheel, check the steel bushing for the same. Then during assembly, wipe them with a high temp lubricant /grease, just enough to provide a water resistant film. Reassemble, and before applying pressure from the brake pedal, try to slide the caliper assy. back and forth. If it's now free to slide, finish the assembly, and take it for a drive. That should (if it was the problem) eliminate the dragging pad, and hot rotor. Let us know what you find.
vettepilot

 

[Alfonso Gomez]

Ok, this has the mechanics stumped. Since the right hand caliper sticks, I had the caliper rebuilt again by a different mechanic. It still sticks. So I ordered from Mid America both front rebuilt calipers and had them installed by a different mechanic. The right caliper still sticks. So he installed a new brake hose, and it still sticks. He checked the brake master cylinder (9 months old) and it is working perfectly. So the mechanic called another mechanic that works for a GM dealer and he said there is a check valve for the braking system under the car that could be sticking. Is this right? If so, where is it located? Is there another check valve? The ABS light only goes on when you start the engine and only for a couple of seconds. Any more advise? Thank you.
This is driving me nuts.

 

[vettepilot]

OK,
Let's try another approach. Since the symptoms are all pointing to the right side caliper, I assume that most or all of the attention has been given to solve the problem on that side. However, there is a slight chance that you could have more than one probem causing the symptom you described.
Begin by checking the steel brake lines that run to all four brake calipers, front and rear. Make certain something didn't crush one or more. This would hamper the equal line pressure to one or more of the calipers, causing lower line volume to one or more vs the others, and could also retain pressure on one side over the other. This would be detected by the ABS system and I think it would try to compensate for it by increasing the pressure to the affected caliper(s).
There is a check valve in the system, but I am certain it is for the entire system, this maintains a low line pressure to help keep the pistons from retracting into the caliper bore, thus maintaining a slight pressure on the rotor by the pads. This does two things. one it helps keep the pads/rotors cleaned of moisture/dirt, and second it greatly reduces the time it takes for the pads to react to pressure input when the brake is applied.
This check valve is integralto the proportioning valve, and it actually performs two functions, the first is as I mentioned above, the second is to provide higher line pressure to the front system as compared to the rear since the front is where the most work is performed and needed when the weight shift goes forward under braking action.
Now since both caliper assemblies have been replaced, I think you can safely eliminate the actual caliper as the culprit.
The proportioning valve that I mentioned is located inside the master cylinder. I have not worked on those early ABS systems, so I am not well versed in their structure and operation. If they are similiar to the later systems of the 90s, then they should be about the same.
To get to the proportioning valve the master cylinder must be removed, and disassembled. The valve is located in the forward section of the master cylinder bore.
Also, the ABS moulator valve could be causing uneven braking action.
Check to make certain the entire system has been bled using the correct procedures, any air in the system could throw the ABS into a tizzy, causing uneven braking. Also that all the fittings to the modulator valve are correctly installed, and not leaking.
Did you just get the car, or have you owned it for a while and the problem just started?

vettepilot

 

[Alfonso Gomez]

to vettepilot

Yesterday I had front rebuilt calipers from Mid America installed. Also both front brake hoses (again). The system was bled and the front right wheel did not turn. That side caliper is still stuck. So all lines under the car were checked and no damage was detected. Another mechanic was consulted and determined that the ABS modulator valve was the culprit.
The guy called a big GM parts dealer and the cost of the ABS unit is a mere $1,912.00:eek :eek :eek .
Now, my question is: Does the whole unit has to be changed? Why not the modulator valve only? The mechanic was told the modulator valve was not sold alone but maybe it is the dealers way of making more money. Can that valve be repaired? How difficult is it to eliminate the ABS system? HELP

 

[vettepilot]

I'm afraid the modulator valve is not one of those things that can be replaced as a single item. The entire modulator assembly must be replaced. It is a very close tolorance unit and not serviceable at the user level as far as I know.
You might check to see if it is available as a re-manufactured unit from your local autoparts store, if it is then you might save some money because there is probably a core charge that will be refunded upon return of the old unit.
I wouldn't remove the ABS because it is a very valuable safety feature, and the entire brake system was designed to function around the ABS system operation.
vettepilot

 

[LUV2DRV]

If you can't get a rebuilt, how about getting one from a salvage yard?

 

[vettepilot]

I was going to mention the salvage yard route also. The main thing you want to watch out for is moisture or other contaminates getting into the modulator assy. If one was taken off a car and the holes were capped ASAP, then it would probably be OK. Otherwise you risk introducing contamination into your entire brake system. As we all know, that is a big :nono
You might want to check around your local area, or go on-line. There are some very good used parts distributors around the country, so you might be able to get through this and still eat steak.
vettepilot

 

[LUV2DRV]

Good point.
There's a C4 salvage yard not far from me that has everything indoors. They even dismantle the cars indoors.
Might be worth a shot. Only a toll free call away.

www.tabercorvetteparts.com

Talk to Kevin if you can.

 

[Topless]

Bone yards....
look on ebay for "fparts" or call them at 1-800-367-8388
Guy Sava is the person I dealt with. They part out vettes. Good people to deal with. They might be able to help.

 

[AnytownUSA]

tkrphxr is thinking the same thing I am. I had a 94 Sonoma that would drag on the right side. Sometimes it would pull during braking but sometimes not. I rebuilt my stock caliper, no good. Replaced it, no good. I finally ended up changing the brake line through the advice of a coworker. That did the trick. The inside lining of the rubber line had begun to deteriorate and would allow flow on compression, but restricted flow on release - acting like a one way valve. Brake lines are cheaper than about any thing else in brakes and even if that isn't it at least you won't have to worry about that in the near future.