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Two years ago, when someone botched a brake job on me (I was expecting pads, and maybe rotors, but it ended up being 4 calipers, a master cylinder, and a warning switch junction too!), I vowed a learn to do all this stuff myself. I subsequently found the Brickboard, and have since done heater motor, trailing arm bushings, struts, etc., and all has gone well.
Today, I took my first shot at brakes - front pads. Followed Bentley; seemed simple enough. Here's some things I did differently:
- after pushing the pistons back to make room for the new pads, I read that you should clean the part of the piston that contacts the shim/pad. I pushed the piston back out by depressing the pedal a bit, but not much happened. I realized I'd have to push a little harder, so I put the old pads back in, to make sure the piston didn't over-extend. I figured it would be the same braking stroke as before starting this job, since the original pads were in. I didn't think about the caveat regarding exceeding the old pedal travel limit on a used master cylinder, which I thought only applied when bleeding brakes, and I hadn't opened the lines. By the time the pistons had pushed out however, the pedal was a lot closer to the floor than before.
- I was careful to keep the silicon grease off of the braking surfaces, but to be sure, I wiped one passenger side pad with brake cleaner on a cloth. I saw that this left a lint on the surface, so I brushed it off with brake cleaner and a brush. I also sprayed brake cleaner on both sides of both rotors at the end, in case there were any grease smears on them.
When finished, Worried now about the pedal travel issue, I did pump the brakes with short strokes to seat the pads. Took her for a test drive, and noticed two problems:
- the pedal seemed really spongy. I also thought it was depressing farther over time, as I was pushing hard, but it never went to the floor or anything.
- at the end of some stops, or when releasing the brake slowly when parked, I got a terrible vibration from the passenger side (low frequency, not a squeal).
Being totally frustrated at this point, I came in to do a post, but then I decided to take it for another extended test drive. At first it seemed a little better, then it pulled to one side, but finally, it seemed to brake quite well. The low frequency vibration went away (although it has now squealed slightly a couple of times from that side), and the pedal does not seem to go down as far now. It does seem spongy, but it might have been like that before I started; maybe I was just focussing on it too much because of the increased travel earlier.
Do you think my master cylinder needs replacing? Is it possible that:
a) crap from pushing in the caliper pistons got into the master cylider seals, but then finally worked itself out (or is there too much brake line length for that to happen)? I've seen recommendations for opening the bleeder when doing this, so the crap goes out instead of back up the lines, but I didn't want to start another possible problem.
or
b) could pressing the pedal past its normal travel once, have gotten the seal dirty, and it cleaned back up over time?
I'm thinking that I should first try doing a brake fluid flush. It needs it anyway, and afterwards with hopefully no spongyness, I should be able to gauge clearly whether the master cylinder seal is leaking at all. The wear-in of vibration and pulling surprised me a bit - or is this normal? Maybe it was due to me wiping the pad with a cloth. Or maybe I put to much silicon on the backs and it squeezed out and got on the braking surface. Would this be "fatal", or would it burn off and be okay?
Any comments or insights are much appreciated.
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Actually, you didn't have to worry about a little rag lint on the linings -- they'll be gone quickly. But you should keep everything clean -- using brake cleaner generously is a good idea.
Also, put silicone grease on:
1) the back of the pads, between it and the little metal plates (antisqueal);
2) on the short edges of the pads, so they can slide sideways in the caliper as they wear;
and 3) on the small pins that hold the pads in place, for the same reason.
Doing what you did about stepping on the pedal to clean the pistons' faces was not good -- you may find that you'll need a new M/C soon, even if it seems good now. If you ever want to extend the pistons, use your pressure bleeder.
All you really have to do for the piston's faces is gently clean it (and you can do that with them full retracted -- you don't have to be a perfectionist with this. But also be sure that the pistons' boots (the rubber ring around them) is clean and intact! And be sure that no piece of that rubber boot (e.g., a fragment) is "caught" between the piston and the back of the pads.
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Yeah, I figured that re: the lint; I was groping for explanations though. Did all the greasing you mention. How thick do you put it on? Just a thin smear, or a coat so you can't see through it?
How did you know I had a pressure bleeder? I suppose if I'd just caused the same amount of pedal travel as it had before the job, I'd have been okay too.
You may have nailed my vibration/pulling issue with your last comment. The reason I extended the caliper piston, was because the boot was sticking out farther than the piston when I tried to clean them while they were retracted. It was probably the same situation when I retracted them the second time, so there was a little rubber between the piston and the pad at first. I guess it dislodged/cut or something, since it's not causing a problem now.
Thanks for the feedback!
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posted by
someone claiming to be Ken C
on
Mon Sep 27 02:32 CST 2004 [ RELATED]
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You don't want too much silicone so that it will migrate to the surfaces of the pads or rotor; but I do like to put it on thick enough to protect the parts' surfaces from corrosion.
The reason I know about the pistons' boots getting jammed is that this was the reason I was once able to buy a really beautiful car for WELL UNDER the market price. I went to see a car that was priced incredibly low -- turns out, the original owner was "disgusted" with the car's brakes. It had been given a "brake job" by Midas, and since that time, the pedal was soft and sunk too low. The owner had insisted on several bleedings, and Midas even sold him a new M/C which didn't help; and then he took it to another shop for bleedings, and yet the pedal was still soft. Lacking confidence in the car's brakes, the owner just wanted to get rid of the car. My test drive confirmed that the brakes were "scary" soft, but I bought it at that bargain price, figuring I might be able to solve the mystery, and even if I had to rebuild the entire system, I'd still have made a steal -- the price was that low!
Well, I brought it home, took off the front wheels first, looked at the caliper, and guess what? The boots of the pistons, on both sides, were jammed between the piston and pad! Obviously, the original Midas worker just pushed in the new pads, rolling the boots over the piston! That layer of rubber was enough to stymie the techs in both that Midas shop and also another one -- they just never looked!
And I came out a winner! I replaced the boots on the pistons, and the brake pedal became rock hard again! And those calipers and pistons, because I flush the fluid every year and put in new pads regularly so that the pistons aren't unduly extended, lasted another 110,000 mi (180,000 mi total) -- and when I did change them, they were still good -- I changed them just "to be sure".
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Methinks you know brakes. The fluid flush may be a bit more obvious, but the comment about "new pads regularly so that the pistons aren't unduly extended" - I think that's brilliant.
I can remember before my fiasco 2 years ago, the mechanic kept telling me my brakes were fine. I thought I was ahead of the game, getting all those miles out of the pads. Had I changed them regularly, it would of course meant that the calipers would have been cleaned up/serviced. With the boots not getting so stretched out, they'd be less likely to crack. Add in your suggested fluid change, and I'd probably be on the same calipers now.
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If it's vibrating or pulling, you most likely got some grease on the pads, easily cleaned with spray brake cleaner.
or you might have cocked the piston in the bore if you didn't compress it evenly.
Did you remove the caliper pins, clean and re-grease them?
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '82 Mercedes 300SD
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I think Ken's point about the boot rubber getting in between the piston and the pad is what happened. If grease does get on the pads, how do you clean them after everything's assembled? Do you just pop the wheel, and spray in from the back of the caliper? Wouldn't the grease tend to soak into the pad, or does it just stay on the surface?
I don't think the piston got cocked, as I used a vice-type spreader between it and the rotor to retract it.
I see Ken's point about caliper pins, but I imagine you're talking about the ones that hold the pads. I used all new hardware, and I did grease them.
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if I get a spot of grease on a pad, I usually just take a couple of large screwdrivers and CAREFULLY pry the pads back just enough for a small air space between them and the rotor, then spray brake cleaner liberally down in there. remembering to spray both sides of the rotor down completely as well.
The grease /cleaner washes off quickly and any cleaner residue evaporates
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '82 Mercedes 300SD
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Actually, he doesn't have "caliper pins", at least not the large ones that calipers of 700/900's have. The latter have "floating" calipers with pistons on only one side, so the caliper has to move back and fourth to apply pressure to pads on the piston-less side; whereas 240's have firmly bolted (stationary) calipers with pistons on both sides.
The only pins that a 240 caliper has are little ones that merely keep the pads from popping out and falling on the roadway :-).
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Actually, the pins do more than just hold the pad from falling out on a 240. True, they do not move the entire caliper around, but if they get too corroded the pads will stick to a specific spot on them and then "spring" back to that spot, forcing further pedal travel to have the pads reach the rotors. Ask me how I know...Grease up that hardwear!
Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/23mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/
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