When to change brake pads?
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  1. #1
    1000+ Posts silverexec's Avatar
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    Default When to change brake pads?

    Hi guys, the pad-wear warning light has just come on in the 505 so I went and bought some replacement pads intending to fit them today. When I took the wheels off and had a look, there's still a fair bit of meat left on the pads (about 10mm) so I thought I'd leave them on for a bit longer.

    So my question is do you have to change the pads as soon as the warning light goes on or can you wait until the friction material gets to a certain minimum thickness before changing them? And if I CAN wait, what is the minimum thickness I should leave them to?

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    Richard
    - Richard

    Now: 405 SRI D70 '93
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    Richard, you can DEFINITELY wait. The minimum thickness is basically about 2-3mm - as long as the friction material isn't through to the metal. This allows for slightly angled wear. The other consideration is heat getting through to the pistons via the pad on a hard driven car if the friction material is too thin, but this usually isn't a problem on a car like a 505.

    The pad wear sensor is basically a metal stud thing with a wire into it that earths onto the disc, completing the lamp circuit - obviously yours were too close to the disc to start with. Or possibly, being 10mm left, there is an unintended earth in it? 10mm isn't far off a new pad!

    FWIW, my wife's '02 Astra has the more primitive wear warning of a metal clip that rubs on the disc and makes a horrible squeaking noise. I just bend these a bit, and this time I got about 6 months more use and they still had about 3mm left.

    Cheers

    Stuey


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    Contented Peugeot Driver addo's Avatar
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    At the risk of stating what you may know, the warning light comes on when a metal contact pin in a hole through the pad/backer makes contact with the running surface. You may have a mis-installed or incorrect indicator contact pin. See if it's both sides by unplugging one at a time.

    Riveted pads need to be changed when the rivets are around 1/32" from meeting the disc. Bonded pads can go much closer. I've taken various ones to 3/32" material remaining. Probably depends on the nexus of budget, spouse and cojones!

    Problems with pushing the limits are - need to add extra fluid to make up for the pedal travel, and if you neglect this, travel of the master and slave pistons into a potentially lipped or corroded area of their bores.

    Incidentally if you have concerns about disc rotor thickness the DBA website has throw-away dimensions for most rotors. I'm not a fan of machining discs with every pad replacement but will sandblast good junkyard rotors and press them ito service... Driveforce and others do cheap rotors for many cars. I mention discs because if you take it to a brake place or many garages, they try the package sell on you.

    Cheers, Adam.

  4. #4
    1000+ Posts silverexec's Avatar
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    Quote Originally Posted by Stuey
    Richard, you can DEFINITELY wait.


    I thought it was a bit of a waste if I had to change them so soon. Yeah, my pads have the stud pushed into the friction material from the top of the pad, and on the passenger side the stud has started rubbing up against the disc. The studs seem to be placed a few millimeters away from the backing plate, and being a couple of mm thick themselves means the warning light would come on even with a fair bit of friction material left. Could this be a ploy to scare people into buying replacment pads sooner than necessary??

    Anyway, thanks for all the info. I'll keep an eye on the brake fluid level and pad thickness every so often to make sure they don't go too low. I got caught out some time ago when my rear pads wore down to metal and it was only when I noticed a grinding noise while braking that I realised they were so low. The pads were replaced and there was no damage to the disc luckily. I don't understand why on 505s there isn't any pad-wear wiring to the back pads but only to the front's. The pads themselves have the earthing stud and wiring but there isn't anything for them to plug into.
    - Richard

    Now: 405 SRI D70 '93
    - 2.0L manual
    Earlier: 505 GTi Executive '85
    - hence "Silver Exec"...
    25 GTX '86
    - manual conversion

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    Budding Architect ???? pugrambo's Avatar
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    change the pads when the light goes back out again

    Addo

    as for topping up the fluid as they wear, don't

    you never need to top up fluid unless you have a leak and even then the leak should be fixed

    the reseviour holds more than enough fluid for the complete braking system in a car with plenty in reserve when everything is at it's maximum

    there is no risk of the master cylinder travelling any further when the pads are new or worn, it travel the same amount each and every time you place foot on brake

    as for the wheel/caliper pistons using part that may have a lip or rust on them is very unlikely unless you have a car that hasn't had any fluid changes in it's lifetime as the further the piston comes out to make up for the pad wearing away it is in the caliper and hence away form the elements
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    Contented Peugeot Driver addo's Avatar
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    If the pads have worn 4mm, and you have twin-piston calipers with 38mm bores, that's an extra 4mm travel. Crunching it gives:

    (3.8÷2)²×Pi×0.4 = 4.536CC per piston; or 18cc per pair of front calipers.

    I know the piston seal is supposed to make the piston return to its free position, so in other words there is greater fluid displacement in a caliper activating a worn pad. How can this be otherwise? If you run disc/drum with a check valve on the rears you'll hold more fluid in that rear line/slaves as the shoes wear. Thus a lower reservoir. I agree this doesn't happen on a disc "circuit".

    Regarding the bore damage issues - surely I'm not the only slacker here who will let things go years longer than ideal? Maybe not tyres, though!

    Cheers, Adam.

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    Budding Architect ???? pugrambo's Avatar
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    Quote Originally Posted by addo
    If the pads have worn 4mm, and you have twin-piston calipers with 38mm bores, that's an extra 4mm travel. Crunching it gives:

    (3.8÷2)²×Pi×0.4 = 4.536CC per piston; or 18cc per pair of front calipers.

    I know the piston seal is supposed to make the piston return to its free position, so in other words there is greater fluid displacement in a caliper activating a worn pad. How can this be otherwise? If you run disc/drum with a check valve on the rears you'll hold more fluid in that rear line/slaves as the shoes wear. Thus a lower reservoir. I agree this doesn't happen on a disc "circuit".

    Regarding the bore damage issues - surely I'm not the only slacker here who will let things go years longer than ideal? Maybe not tyres, though!

    Cheers, Adam.
    not doubting the maths but the brake system is a hydrualic circuit and as such is designed with a reseviour that is able to hold enough fluid with enough in reserve to allow all the slaves/wheel cylinders to be at max travel and have enough fluid left in reserve for the system to work and to have a safe working level

    topping up fluid is fine but i have seen it many times when people have added fluid as things wear to then forget when putting pads in to remove the excess and have fluid flow out of the reseviour

    i change my fluid every 2-3 years as i am sure most who service their own cars would
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  8. #8
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    At the point of the light coming on is the time to buy new pads and start to think about when you can get around to doing the job.
    You do not want to leave it until the last mm of useful wear as a good gouging of the disc by the metal backing is a PITA.
    I would suggest that you get the wheels off and have a good look at the calipers/rubbers and other associated bits, to see if there is anything that may need doing while you are there, (slides on the front are a good one to see if the cups are RS and there is a need to clean and lube the slides. (reading back I see that you have done this already).


    I would change the pads at the next convenient time and not when you have to because there is no stopping left in the system.
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  9. #9
    mlb
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    Richard,

    I had a similar thing a couple of months ago. Brake light came on and stayed on. Lashed out and bought new pads. Took the wheels off and found that there was stilll 10mm of pad left. What had happened is the wire from the wear sensor had been rubbing on the inside of the wheel and took the insulation off, hence it was earthing on the wheel. Wrapped in insulating tape and remounted it - no more light.

    Might be in your case.

    Matt
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  10. #10
    Real cars have hydraulics DoubleChevron's Avatar
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    Quote Originally Posted by Gamma
    At the point of the light coming on is the time to buy new pads and start to think about when you can get around to doing the job.
    You do not want to leave it until the last mm of useful wear as a good gouging of the disc by the metal backing is a PITA.
    I would suggest that you get the wheels off and have a good look at the calipers/rubbers and other associated bits, to see if there is anything that may need doing while you are there, (slides on the front are a good one to see if the cups are RS and there is a need to clean and lube the slides. (reading back I see that you have done this already).


    I would change the pads at the next convenient time and not when you have to because there is no stopping left in the system.
    You don't run out of stopping, you just get a god awful screaching noise of metal against metal.

    I've been caught out before where the pads looked new, however there was a lip at the top of the pad about 1/4mm wide. When I removed the pad there was nothing left of it (I could have scratched the last of the friction material off with my finger nails). That bloody lip made anyone taking a quick glance at the caliper think "Goodie new brake pads, I will not worry about them ).

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  11. #11
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    Rambo, the missus' Astra had that problem. Someone had refilled the fluid and when I pushed the pistons back, I had to use a big 'horse' syringe to remove about 50ml of fluid. I think dealers etc. like to top it up otherwise it looks dodgy in the customer's eyes.

    Gamma, 10mm of friction material is about a year's worth of wear. I don't think there's any hurry. Stock pads start with about 12mm depending on the type.

    Stuey


    2003 PEUGEOT 206 GTi

  12. #12
    1000+ Posts silverexec's Avatar
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    Quote Originally Posted by mlb
    the wire from the wear sensor had been rubbing on the inside of the wheel and took the insulation off, hence it was earthing on the wheel. Wrapped in insulating tape and remounted it - no more light.

    Might be in your case.
    The stud in the pad was definitely rubbing up against the disc in my case, all the wiring seemed OK. To stop the warning light coming on each time I pressed the brakes, I disconnected the wiring from that pad... Don't worry, the other pad will wear down soon enough and the light will come back on to remind me again. I'll take the wheels off and check out the pads again when that happens.
    - Richard

    Now: 405 SRI D70 '93
    - 2.0L manual
    Earlier: 505 GTi Executive '85
    - hence "Silver Exec"...
    25 GTX '86
    - manual conversion

  13. #13
    1000+ Posts Gamma's Avatar
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    Quote Originally Posted by Stuey

    Gamma, 10mm of friction material is about a year's worth of wear. I don't think there's any hurry. Stock pads start with about 12mm depending on the type.

    Stuey
    True,
    I would however contend that a good exercise run down the Oxley Hill would deal with 5-6mm easy.

    I go down the line of, by the time you get your act togeather to change them, the pads, they really, really need changing.....
    /// 1986 SII 505 GTI
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