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Author Topic: disc brake -- new pads  (Read 1326 times)

Mike_D

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disc brake -- new pads
« on: March 13, 2013, 10:59:37 PM »
So I tackled a big job today and replaced my fork seals (a great thanks goes to baird for all the info, much help!).  The weepy forks were getting oil onto the disc brake thus making it inoperable over time (well, it stopped about as good as a front drum).  SO, I also put new pads in the disc.  And now the front wheel spins but with restriction.  I pushed the pistons all the way back in like the manual says to do.  When i first mounted the wheel it spun fine, no problem, just like before.  But the first time I used the lever it pulled all the way in with no resistance then pump, pump, pump its back to normal but now the wheel is restricted by the new pads.

Any ideas?  This was the last bit of a long, long day so I headed home but would like to get it all sorted tomorrow.  Should I just do it again?

I have no experience with Disc brakes (though they seem simple).  I actually have an Iron Barrel but the previous owner put this on:

http://nfieldgear.com/enfield-store/disc-brake-kit-including-wheel.html

Any help?

tooseevee

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Re: disc brake -- new pads
« Reply #1 on: March 14, 2013, 04:04:42 PM »
    You lost hydraulic fluid (where?) when you pressed the pistons in (or so it seems) & now when you press your brake lever you're squeezing air, not pushing on fluid.

     Your brakes need to be bled.  You must either, (a) find someone who's done it before & invite him over or, (b) get a manual & read how to do it (tricky the first time) or, (c) someone on this forum may give you step-by-step direction. I, personally, won't (no offense) do it. Sometimes the fronts are very tricky & every individual bike can be different. Other times it goes slick as shit, no problem. It seems random & without reason so I'll beg off. I don't want anything to do with "bleeding brakes by internet" especially the first time.     

   
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barenekd

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Re: disc brake -- new pads
« Reply #2 on: March 14, 2013, 05:24:42 PM »
Clean out the gap between the pistons and calipers. Dirt in there can restrict the movement of the pistons not allowing them to retract back off the disk like they should. A spray can of brake cleaner should do the job.
You may need to pop the pistons out and replace the Orings.
Bare
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Arizoni

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Re: disc brake -- new pads
« Reply #3 on: March 15, 2013, 12:22:32 AM »
That could be needed but I wouldn't remove the pistons unless everything else had been tried.
Usually the seals will last for years unless the wrong fluid has been put into the system.

If the brake lever comes to a hard stop when it is squeezed, no air has entered the system.  If it feels squishy or spongy the system needs to be bled.

When the new pads are installed, a very light coating of special silicone disk brake grease should be applied to the two pins before they are inserted thru the caliper body and the two brake pads.  Failing to do this can cause the disk pads to bind up and not to retract from the disk.
Don't use ordinary grease to lube these pins.  If you don't have the right stuff it can be bought at a auto supply store and it doesn't cost a lot.

Another cause of the pads to hang up is if the spring is not installed correctly but this is usually noticeable when the pads are installed.

I know of one case where the hydraulic system somehow got too much fluid in it.
That also caused the pads to fail to retract but if that was the case here I would expect it to have been a problem before.
In any case, to elimanate this potential problem, remove the small rubber cap on the brake caliper bleed screw.  Use a wrench to unscrew the bleed screw 1/2 turn. 
If fluid squirts out, you have found the problem.  If fluid just oozes out then this was not the problem. 
In either case, after a small bit of fluid has come out, tighten the screw and reinstall the rubber cap.  Do NOT get that brake fluid on any painted surface.  It will soften and eat the paint.

Let us know what you find. :)
Jim
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1999 Miata 10th Anniversary

AgentX

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Re: disc brake -- new pads
« Reply #4 on: March 15, 2013, 09:45:22 AM »
    You lost hydraulic fluid (where?) when you pressed the pistons in (or so it seems) & now when you press your brake lever you're squeezing air, not pushing on fluid.

If his brakes feel as solid as they did before, which seems to be the case from the description, I highly doubt he let air in his system.  Nor does anything he describes doing seem like an action which could have let air into the system.

Can't say what's actually wrong, but like Arizoni and Bare I'd suspect an issue with the seals or installation of the pads and spring.  And the most likely thing to go wrong is what you just messed with...installation of the pads, springs, and retaining hardware.

The pistons should retract after the pads make solid contact if the seals are working correctly; if they are worn out maybe they won't pull back on the pistons properly.  But it's less likely they chose just this moment to go bad compared to the prospect of it being a recent change you just made.  That's where I'd look first.


I've had mountain bike disc brake pads come out of the package too thick; they'd trap the rotor and hinder free rotation once installed, but in those cases there wasn't any empty pumping of the brake before the pistons made contact...as soon as you pulled the brake it was solid and the pads couldn't retract any more as the pistons were already flush inside the caliper.  Don't suspect that in this case.


Oh, and if your old pads are lightly contaminated but still otherwise useful, you might try baking off the contamination.  You can probably find recommended heats/times in a google search.  375 and until they stop smoking is my recollection.

Mike_D

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Re: disc brake -- new pads
« Reply #5 on: March 15, 2013, 02:08:24 PM »
Thanks for all the help guys.  Gonna get some of that disc break grease and go back in and take a look when I get a chance.  Hopefully soon.  I'll report back with my findings!

Arizoni

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Re: disc brake -- new pads
« Reply #6 on: March 15, 2013, 10:12:39 PM »
Actually, the pumping up that was needed is normal when the pads are changed.

When the pistons are pushed way into the cylinders it forces the brake fluid back up the hose and into the master cylinder.
Then, when the new pads are installed, the pistons are still sitting back in the holes and they are not contacting the back of the pads.

At this stage, part of the pad replacement proceedure is to pump the brake lever to push the pistons back out until they stop against the back of the pads.  Once this is done the pads will be contacting the disk and some resistance to the wheel rotating is normal until it has been ridden a few blocks.

Riding the motorcycle should give several opportunities to use the brakes which usually is enough to allow the pads to align and to start to wear in with the disk.
 
By the way, speaking of wearing in with the disk,  IMO it is a good idea to roughen up the surface of the disk by sanding both sides of it with some rough black silicone carbide sandpaper.
The scratches produced by doing this should be quite noticeable.
 This sanding should be in a radial direction.  That is,  in the direction of the axle and tire rather than circumferentially in the direction the disk rotates.
 
The roughened disk will help the new pads wear a bit so they will mate properly with the disk.

There is nothing on the piston or seals that would retract the pads from contacting the disk when the brake lever is released.
The slight wobble of the disk while the bike is underway is all there is to push the pads away from the disk and a few thousandths of an inch is normally all that is needed or desired.

As I mentioned, failure to lubricate the pad pins or some residual pressure in the brake hose is about all that can keep the pads from being moved the few thousandths needed to allow the disk to rotate freely.
Jim
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1999 Miata 10th Anniversary

AgentX

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Re: disc brake -- new pads
« Reply #7 on: March 16, 2013, 01:31:55 AM »
Actually, the pumping up that was needed is normal when the pads are changed.

...
There is nothing on the piston or seals that would retract the pads from contacting the disk when the brake lever is released.

Yes, of course, if it was I who implied that the re-pumping of the brakes was abnormal I didn't mean to.

But in my understanding, the piston seals actually do retract the pistons when you release pressure on the lever.  As the piston passes through on its way towards the rotor, it bends the lip of the seal slightly in towards the rotor as well.  When you release the lever, the force of the seal returning to its normal, un-bent state helps retract the piston from the lever.

howstuffworks.com (always a reliable source) cites both rotor wobble and the piston effect together:

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


Self-Adjusting Brakes

­

­The single-piston floating-caliper disc brake is self-centering and self-adjusting. The caliper is able to slide from side to side so it will move to the center each time the brakes are applied. Also, since there is no spring to pull the pads away from the disc, the pads always stay in light contact with the rotor (the rubber piston seal and any wobble in the rotor may actually pull the pads a small distance away from the rotor). This is important because the pistons in the brakes are much larger in diameter than the ones in the master cylinder. If the brake pistons retracted into their cylinders, it might take several applications of the brake pedal to pump enough fluid into the brake cylinder to engage the brake pads.

However, in my best guess, a problem in the piston seals big enough to cause a lack of retraction would most likely be accompanied by a rather obvious seepage of fluid around the pistons.

Mike_D

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Re: disc brake -- new pads
« Reply #8 on: March 20, 2013, 07:09:43 PM »
OK so it seems like it was the new pads.  Upon closer examination it appears like they are a little bit different than my old ones where the pins go through.  Its so slight but it seems to be binding near the pins.  I lubed them all up to no avail.  Cleaned up the old ones real good and popped em back in with no problems.  The bike stops better than before (though maybe not quite as well as it could) so the old pads will have to do until I can get new ones.  Let's just say that the old pads didn't come from our host and that I learned my lesson and will buy my new ones from a more reputable distributor!

Kevin Mahoney

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Re: disc brake -- new pads
« Reply #9 on: March 20, 2013, 08:30:50 PM »
Unfortunately there are a lot of what RE calls "spurious" spares around, especially from India. ( No offense intended to our Indian friends but this one cannot be blamed on the Chinese LOL). You especially see a lot of them on EBay. They are advertised as OEM, still in the RE bag, comes from the OEM supplier out the back door, better than etc. The reality of the Indian street market is that price is everything and not quality. We get offers for this stuff literally everyday. I have yet (15 years) to find any of this stuff "as good as". Filter are not for certain, gaskets are not (many of the UCE gaskets are made in the US), brake products are not (and are likely to contain asbestos) etc.  I have been in probably 100 RE aftermarket stores in India and only have one that I will buy from. Alan Hitchcock and I have even done this together in India and we also share war stories. Even with our one good supplier we have to inspect very carefully and our reject rate is higher than you want to know about. The other big problem is who are you going to call for a refund or new part when something is "half a hole off"?
Genuine RE spares are cheap relatively from your dealer or us. Some of you have had good luck with overseas spares. But it is a case of buyer beware. Even the good guys  send junk sometimes because it is perfectly acceptable in India if the price is good.

AgentX

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Re: disc brake -- new pads
« Reply #10 on: March 21, 2013, 08:19:38 AM »
I bought the high-performance EBC pads from NFieldgear and am very happy with them.

High On Octane

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Re: disc brake -- new pads
« Reply #11 on: March 21, 2013, 03:54:15 PM »
I bought the high-performance EBC pads from NFieldgear and am very happy with them.
EBC makes a really good pad.  I've had good luck with them in the past.  I put a lot of miles on my EBC pads on my 83 Suzuki and they worked great and never faded out on me carving the roads up in the Rocky Mountains.
 Scottie
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