Dragging brake caliper

[labman]

quote:

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Originally posted by John Hilmer:
Woha, I didn't mean to start a war over grease and brake fluid, sorry.

The Toyota manual shows two versions of rebuild kits. One supplies "Lithium soap base glycol grease, for all sliding surfaces." and the second supplies none. In the second case they state to soak the piston in break fluid prior to installation and to use silicone based or high temperature multi-purpose grease on the pins.

Regarding the drain and refill issue they state to do this if any contamination is seen. This brings up a new question for me.

What causes the fluid to become black. The wifes '96 Camry is black so I'll change that out, but what is the source that causes it to get like that? It seems a waste to just drain and replace the fluid if there are seals/lines etc that are decomposing and coloring the fluid. If it is soil ingress then I assume the caliper seals and the like need replaced.

You folks have been a big help to me so far, a bit more help here would be much appreciated. John

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One of the reasons I like to rebuild my calipers myself is to make sure they go back together with a good coat of Sil-Glyde on everything and not some hygroscopic crap like brake fluid or a glycol soap. Only bean counters would use something like that. At least here in the salt belt, one of the main brake problems is corrosion of the outer part of the piston. It doesn't see the brake fluid from one pad change to the next. What is on the the outer part of the piston, is mainly what it was lubed with on assembly. It has nothing but the boot to keep the salt, moisture, and grit away from it. The chrome plating is abraded as the piston slowly works it way out of the seal. One more time guys. How does changing the fluid protect the part of the piston beyond the seal?

Nice somebody else has looked at a brake caliper and realized flushing the fluid may not push the crud up out of the bleeder. I also question how much of the old stuff in the narrow clearance between the piston and bore is flushed out.

Brake fluid that has extracted a little carbon black filler from the rubber parts will continue to work fine. It is like changing the oil in a diesel as soon as it turns black from soot.

Notice how what was likely a purely mechanical problem external to the caliper fluid has turned into an insistence on the useless changing of fluid.

 

[TxGreaseMonkey]

John Hilmer, I recommend a Cal-Van No. 247 Brake Bleeder Economy One-Man, which costs $2.99. It has a one-way check valve in it and allows me to change the brake fluid in 45 minutes, working by myself. I use a battery squeeze bulb ($2.50 from Advance Auto) to first suck out and replace the fluid in the master cylinder reservoir. I use Valvoline SynPower brake fluid and ATE TYP 200, both of which have high wet boiling points.

 

[Tosh]

quote:

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Originally posted by John Hilmer:
I'm also considering a modification to the calipers that would add a bleeder port at the bottom of the caliper so that in the future I can bleed the caliper from both the bottom and the top. I can do this while the rotors are being turned with little trouble.

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I think this would be a very useful maintenance modification if you were keeping your car for a very long time and weren't able to regularly replace the fluid. But from a liability standpoint, you would expose yourself to unwanted attention in the case of a brake-related issue. You probably know what you're doing, but as soon as you modify a safety-related component, you will give the manufacturer an opening to blame you if their brakes fail for any reason. You're better off replacing the fluid every 2-3 years. It doesn't get out all the glop, but it replenishes anti-corrosion additives and removes some old fluid. (Take that, Labman!)

 

[John Hilmer]

Labman, TxGreaseMonkey, Tosh,

I take your experiance and advice as offered Labman, it is much appreciated and seriously valued. Can you appreciate that this is my wifes car and I would prefer to see clean fluid in the system. Anal on my part perhaps, but it makes me feel better. For the $4.59 and an hour of fussing around it makes me more assured and I sleep better. The Sil-Glyde is in hand and ready.

TxGreaseMonkey, I'll look into this, it seems a better way to deal with the process given my recent experiance, thanks.

Tosh, You are right about the liability issues and I'm probably not going to the trouble of doing this, but not because of those troubles.

Labman pointed out that the extended portion on the pistons are exposed to the un-filtered environment. This is true and sorta implies that soil and abuse is not something that can be avoided. I need to think through this a bit more, but he has made a significant point I think.

Best To All, John

 

[labman]

quote:

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Originally posted by vizvo:
For the record, the force for retracting the caliper piston is primarily provided by the lathe cut seal in the caliper bore. It is designed to flex when the piston is applied, and to pull back on the piston when the piston is released. It only provides a mere fraction of an inch of movement, just enough for the piston to clear the rotor. These seals can age and lose their spring over time. They might continue to seal fluid but they can lose their resiliency. snip....

Another common common cause of brake drag is a misadjusted brake pedal push rod. There should be a slight amount of clearance between the push rod and the master cylinder piston, say about 1/10 of an inch. snip... If you don't find any freeplay at the pedal, then your push rod clearance may need to be adjusted.

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Interesting. Could be. Still, it would take movement to wear the plating thin under the seal. And usually the pistons go bad right in the area where the seal rode as the piston works it way out as the pads wear. Certainly supports my contention that the calipers should be replaced/rebuilt at least every few pad changes. Don't remember a leaking caliper that the piston was in good shape.

You are right on the pedal play. Usually it comes correct from the factory, and won't be a problem unless you replace the MC or booster.

 

[John Hilmer 2]

Yes, this is interesting and makes some sense. The seal could well act to provide a bit of retraction, but I keep thinking of the force required to move the piston. I couldn't do it with hand pressure and had to resort to a "C" clamp. If the seal does this I would think that I could measure it with a dial indicator. It seems easy enough to test. If on the other hand it is the minor run-out due to bearings etc as postulated earlier then rotating the wheel might show some movement of the pads.

In either case I agree with Labman that the failure mode is as he has suggested, and also corrosion behind the seal where contamination is likely to be concentrated.

I may try to set this up and make some measurements. It could be quite interesting to see if there is any thing to learned by this.

 

[vizvo]

For the record, the force for retracting the caliper piston is primarily provided by the lathe cut seal in the caliper bore. It is designed to flex when the piston is applied, and to pull back on the piston when the piston is released. It only provides a mere fraction of an inch of movement, just enough for the piston to clear the rotor. These seals can age and lose their spring over time. They might continue to seal fluid but they can lose their resiliency. Combine this with loss of lubrication on the slides, and contamination of the fluid, and you can get some drag.

Another common common cause of brake drag is a misadjusted brake pedal push rod. There should be a slight amount of clearance between the push rod and the master cylinder piston, say about 1/10 of an inch. This is multiplied by the mechanical advantage provided by the brake pedal which shows itself as freeplay at the pedal. So a break pedal with a mechanical advantage of 4 or 5 and clearance of 1/10 of an inch at the the master cylinder would provide about 1/2 inch of freeplay at the pedal. If you don't find any freeplay at the pedal, then your push rod clearance may need to be adjusted.

 

[vizvo]

For future reference, let me make clear that the fact of the lathe cut bore seal providing the retracting force for the caliper piston is the canonical textbook answer. I take no credit for thinking it up. That is the answer you want to give if you ever take the ASE A5 brake certification test. That's the answer they're looking for.

I'm sure there could be other forces at play. We know that lack of lubrication plays an opposing force, as will a blocked compensating port in the master cylinder. It seems possible that rotor runout, vibration, slight pressure differential, etc., all might play some role in aiding piston retraction too.

 

[John Hilmer]

Both the ECHO and Camry now have rebuilt clipers. Both are functioning very well. They both do retract just ever so slightly but sufficient to greatly reduce pad drag.

Regarding the "Lathe cut" I've got that pegged now. The seal that rides in this recess is nearly square in cross-section. The sides are parallel but the bottom of the recess is deeper in the direction of force than it is at the back. This aides in installing the piston and may act as implied by vizvo's addition to this thread above. I'm still not totally convinced about this as the pisons on both cars are chrome plated and the grease? supplied by Toyota for these applications is really slick when applied. Still, I can visualize the application of force in this situation causing the seal to do what has been suggested.

I didn't measure the return motion as I had wanted to. By the time every thing was done and I was about to put the wheels back on it was late, I was too dirty to handle the camera and I was physically shot. I did put a couple of lug nuts on to hold the rotor in place, and had my partner apply and release the breaks. I felt of the pads as this was being done and could feel the pads move. How much is just a machinist guess but I would say about .008 - .012" full travel. This was static of course. I did measure the rotor run-out and of all four wheels the worst was .003 the best was .001 as measured 1/2" in from the outer edge as Toyota specifies. That doesn't seem to me to provide much in the way of pushing the pads back.

So, there it is from my experiance. I hope it might help someone else in some way.

The milage on the ECHO is up. I need a couple more tanks to be able to quantify this. I'll be tracking the Camry as well because it had lost about 1 MPG over the last year and I'm hopefull that with the fresh rebuild of the calipers that this might also improve.

As an aside, I had been worried that I had a damaged cam or main seal as there has been a minor leak on the passenger side of the Camry which is nearing the end of the second A-RX rinse cycle. Those areas are not leaking now that I have the area cleaned up.

It turns out the the power stearing pump and/or lines are the culprit. I suppose that at 130K that old pump is probably due to be replaced, and that is better than having to deal with the seals on the motor.

Oh, for Labman, the dust covers on the Toyotas cover the protruding piston active area. If the piston dust covers are in good condition there isn't any soil there to damage the piston, cylinder or seals. I found no evidence of soil ingress on any of the four I just worked on. I didn't find any evidence of moisture or other corruption in the cylinders of the calipers either. The camry was treated to new break lines and all new dot 4 fluid. I know, a waste, but looking at the nice clear master cylinder makes me smile.

If anyone is about to replace breaklines let me add that the portion of the threaded fitting that attaches to the line flares whan set tight agains the flared end of the hard line. This makes removal difficult as it drags on the internal thread of the old flex line. File or swadge this down carefully before attempting to attache it to the new lines. If you don't do this it is nearly impossible to get the thread started because the enlarged portion is interfering with the fresh thread in the new flex line.

Thanks to all for the education and help. I got through this with a minimum of cursing and I now understand this system much better. John