general Brake question
 

why do some brakes/pads decide only to use a patch/strip around a disc and not the whole area ?? :shrug:



one of my rears only uses a 1 inch wide area right in the middle :duh:


C

either the pad or disc is "bowed" or has a high/low spot?

I am surprised that wheel passes the mot as I would expect the brake performance to be poor on that wheel

macafee2

It might be OK on the other face which may be enough to scrape through the test.

Suggests that perhaps there's a problem with the top and bottom arcs of the current pads not touching the disc, or there was a problem with the centre arc of the previous pads (so the top and bottom did wear the disc down leaving the centre arc proud).

Without machining the disc and the pads to get them all level, changing both seems the obvious answer.

Mine have always been like that, never a problem with passing MOT's it is normal for the 75 rear brake discs.

The first thing I'd do Chris is remove the sliding caliper, thoroughly wire brush all the contact surfaces to remove the inevitable corrosion, and sparingly apply a suitable lubricant (which isn't going to find its way onto friction surfaces).
Whilst you're at it, check that the piston moves freely when the brake pedal is depressed in a controlled manner.

Simon

Not really NORMAL Harry.---On a vehicle not used often the disc can start to rust. This can make a part of the disc abrasive and will wear the pad material away and the rust will not polish off. As less of the surface of the pad is in contact the pressure gets higher and higher eventually removing the rust. You are then left with only a fraction of the disc providing any stopping power. The MOT station was remiss in giving the braking system a pass.--A visual inspection should be used and not just the dial on the brake tester.

Also the pads will wear faster due to the excessive pressure. Altogether not a good thing. ( Can even lead to brake failure due to over heating when descending a long steep hill for instance. )----:eek:

if you look at alot of these pads , and pay careful attention to the spring clips that hold them inside the piston, more often than not , they are either not positioned centrally or equal. i have noticed this times on different makes. i had one set that would go in reluctantly , and pop back out at the slightest tap on the calliper. sometimes the springs were twisted at an angle or not even the same sizes . i wonder if the clips push out the pads at an angle whilst driving and then re seat when the pedal is applied. some of the new ones are a pain to fit inside the piston and must distort once inside , to an aukward angle and then not give a square initial contact at all. you can try to adjust them equally with the help of some moles but never get them equal as the spring material is very tough. the rear brakes never get much pressure on them as most of the braking is done on the fronts. sometimes the central rivets are not even central to the pad, and vary even on expensive pads. this would also give you problems on the pad carriers with the pads sticking. things aint what they use to be is an understatement... :}

When in use the pads are lightly touching the discs.---It would be impossible for them to twist. Virtually zero clearance.

please read my post again mr colvert. .... :D the o.p was asking about uneven pad wear. i was suggesting in my post that the pads whilst the vehicle was moving were actually springing out at an angle and could possibly be in contact with the discs at an angle thus wearing unevenly. this would also cause the pads to not sit squarely on the metal carrier guides. the pads wont move out of centre once they are up to the piston whilst being applied , thats true, but do we know that they are , and that they are fully square to the piston ? it would only take a very small amount out of square to effect them. ... :D

the springs are quite capable of pushing back a piston. provided of course it moves freely and is not sticking or seized. sometimes , as i have said in a previous post they can take quite a bit of forcing into the piston, no matter how square you try to offer them in.... ;)