Chevy Brake Problem

[barbarossa]

Common problem. The pistons in the calipers are made of a phenolic material and after a while they tend to swell and stick in the caliper bore stick and not retract. I just had this issue on my 07 Suburban. Change the caliper and be aware it could happen on the other 3. I also had it happen on an 85 Pontiac I owned.

Thanks, that makes sense now.

Phenolic, huh? They made an essential part in a system which routinely gets very hot out of plastic? What could possibly go wrong?

 

[buttanic]

Thanks, that makes sense now.

Phenolic, huh? They made an essential part in a system which routinely gets very hot out of plastic? What could possibly go wrong?

Heat is the reason they make it out of phenolic to block heat from the pad getting to the caliper and boiling the fluid..

 

[barbarossa]

Heat is the reason they make it out of phenolic to block heat from the pad getting to the caliper and boiling the fluid..

Well, I'll be...

Learned something new, thanks!

 

[Bam Bam]

I'm no expert, but the big difference is the new pads make the piston run deeper in the caliper, where it hasn't been in a long time, where corrosion and gunk may have formed. Also the seal has to double back over itself with the new pads, another thing that hasn't happened in a while.

Sounds like a caliper issue. The only problems I have had with those brakes have been after replacing the emergency brake shoes and disturbing the cables.

This!! It happened to my truck and I'm a mechanic too, he hit nail in the head! The calipers are aluminum which corrode easier! Time for new ones, I got mine from O'Reilly!

 

[paddle007]

Quiz time.
How many of you knuckle busters know what returns the piston away from the rotor?

 

[buttanic]

Quiz time.
How many of you knuckle busters know what returns the piston away from the rotor?

Nothing. As long as the caliper pistons are not sticking the pads are always in contact with the discs but no pressure is being applied.

 

[paddle007]

Nothing. As long as the caliper pistons are not sticking the pads are always in contact with the discs but no pressure is being applied.

But we just applied over a thousand pounds of hydraulic pressure. Something returns the caliper piston just a little.

 

[barbarossa]

Quiz time.
How many of you knuckle busters know what returns the piston away from the rotor?

The seals?

Possibly also a very slight compression and rebound of the pads?

I'm guessing here.

 

[buttanic]

But we just applied over a thousand pounds of hydraulic pressure. Something returns the caliper piston just a little.

But to pressure goes back to zero when you release the pedal pressure. The pads just lightly drag on the disc.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake2.htm

 

[general mills]

^^^^^^^^

 

[general mills]

FWIW, you may want to try exchanging the pads, seems interesting that it works with the old ones and not with the new. I had pads bad from the factory, they were difficult to get on, and the actually measured a bit out of spec, and did what you are describing. That being said, I think it is more likely that you have a bad caliper. But if you exchange the pads anyway, you won't be driving on the pads you heated up and glazed over in the end.

 

[general mills]

But we just applied over a thousand pounds of hydraulic pressure. Something returns the caliper piston just a little.

friction of the rotor spinning in the pads?

 

[edman87k5]

Not sure on the technical terms, Google fluid dynamics. The fluid doesn't compress so the piston goes outward under fluid pressure, that same fluid draws the piston back when there is no pressure present (assuming the system is air tight). Pads always drag the rotor on discs, that's why old drag racers used drums, you can adjust to zero drag.
On the phenolic Pistons, I don't recall ever seeing one. I have changed countless calipers on more vehicles than I can name and have only seen cast aluminum or different kinds of cast "pot" metals. I have changed a few on later model gm trucks - same issue as described originally and don't recall seeing plastic Pistons in any of them. Not saying they aren't using it now, but why? Sports cars, atvs, motorcycles, aftermarket very expensive brake kits, etc- every one I have ever messed with had metal of some sort for the piston.

 

[buttanic]

Not sure on the technical terms, Google fluid dynamics. The fluid doesn't compress so the piston goes outward under fluid pressure, that same fluid draws the piston back when there is no pressure present (assuming the system is air tight). Pads always drag the rotor on discs, that's why old drag racers used drums, you can adjust to zero drag.
On the phenolic Pistons, I don't recall ever seeing one. I have changed countless calipers on more vehicles than I can name and have only seen cast aluminum or different kinds of cast "pot" metals. I have changed a few on later model gm trucks - same issue as described originally and don't recall seeing plastic Pistons in any of them. Not saying they aren't using it now, but why? Sports cars, atvs, motorcycles, aftermarket very expensive brake kits, etc- every one I have ever messed with had metal of some sort for the piston.

Unless you pop the piston out of the caliper they look like metal because they have a metal face crimped on where they contact the pad.

Caliper pistons may be aluminum, steel or phenolic (plastic). Each type of piston has certain advantages and disadvantages. Aluminum pistons are lightweight but vulnerable to corrosion. Aluminum also transmits heat from the pads to the brake fluid inside the caliper, which can increase the risk of fluid boil and brake fade if the brakes get too hot. Steel pistons are the strongest type of piston, but like aluminum are vulnerable to corrosion (rust). Steel pistons are usually plated with nickel or chrome to improve their resistance to rusting. Steel pistons also transmit heat from the pads to the brake fluid in the calipers. If an aluminum or steel piston corrodes, it may stick in its bore, preventing the caliper from applying the brakes or from releasing the brake once it has been applied. Phenolic pistons are made from cast resin and are lightweight. Phenolic pistons will not corrode but they can absorb moisture and swell, causing them to stick in their bore if the manufacturing clearances are not correct. Phenolic pistons do not conduct heat like metal pistons, which reduces heat transfer from the pads to the fluid in the calipers and reduces the risk of fluid boil and brake fade when the brakes get hot.

 

[edman87k5]

Learn something new every day.

 

[paddle007]

A Raybestos rep taught us it is the square cut seal/o-ring in the caliper. The inward edge rolls inward and when pressure is released it relaxes just a c**t hair. Of course their recommendation was to replace calipers every brake job. My sales skills are not polished enough for that.
The one thing to do every brake job is to lube the slide pins with the proper caliper grease and flush brake fluid.
As far as steel versus phenolic pistons go, never use steel in an extreme environment like Colorado mining roads. I've seen melted locking hubs and center caps. Watched a friend boil his rear brake fluid on a KLR650 on a long downhill. Yes we are lightweights.

 

[general mills]

I always flush out old brake fluid. I never lube. I dress up any contact points and roll with it. It seems as if grease would just collect dirt and dust. I can say, the only car I ever applied grease to is my old fiat spider, thought I would do it by the book. It was squealing by the end of the week. Never had a car do that before.

I would bet that seal has little to do with releasing the brake pad. It may preform better, but would still work. I think it has more to do with having lots of pressure with the pedal pushed, but 0 PSI when you let off. Why would it not release?

Just my thoughts, I am no engineer or caliper salesman.

 

[paddle007]

No grease on the back of the pad. A dab at each end of the pad where it rests against the bracket/contact points and if a full floating caliper the slide pins. If brakes squeal it gets another set of pads.....if the rotor finish is good. Now with all this Chinese made iron it is hit and miss.

 

[SVTFreak]

FWIW, you may want to try exchanging the pads, seems interesting that it works with the old ones and not with the new. I had pads bad from the factory, they were difficult to get on, and the actually measured a bit out of spec, and did what you are describing. That being said, I think it is more likely that you have a bad caliper. But if you exchange the pads anyway, you won't be driving on the pads you heated up and glazed over in the end.

It does this because the new pads are thicker and put the piston back into a spot that isn't smooth on the cylinder wall. Not a pad problem.

As far as what returns the piston? Duh, everyone knows that's the piston return Springs!

 

[Creoleman]

Barbarossa, make sure you don't have a collapsed hose on that side, as I did years ago, with a '93 Chevy P/U I had. It caused so much heat that it permanently burnished the alloy wheel that was on that side. Apologies if anyone already suggested this.