Brakes come on without breaking
- Thread starter Sameccles
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slob
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are you sure its your brakes and not somrthing else causing you to slow down?
chabrolly
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How Did You Know That Are The Brakes .. Is There Any Sound ? Squealing ? Maybe The Problem Is Sothing Else .. Or It's A Miracle ..
slob
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i sed that
I know it is the brakes because the wheels won't turn and the brakes are "rather warm"!
rick071155
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hiya, the brakes will get "rather" warm just by normal use mate , is it "the" brakes or "a" brake are they all roughly the same temperature ? or do you have one hotter than the others , my old classic developted this problem it turned out to be the o/s caliper siezing up .
rick
rick
Teflon
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A bit more info could be usful; RRVogue Tdi... possibly holes in my knowledge of the model variations; but is that a classic, a P38 or 'New' RR? Feeling its a classic, but like to be sure.
What'goodies' has the car got on it? Is it ABS, does it have TRaction Control or Hill Decent Control of anything like that on it, that work through the brake system?
Brakes wont generally try and apply themselves; unless there is a leccy brain and pump tthat might try and do that for TC. If there's leccy intervention...... brings in a whole remit of pottential causes to look at.
So, conventional system, without any microchips in it; not being patronising; but going back to 1st principles;
Pedal, end of pedal a cylinder, pipe from cylinder to caliper with another cylinder in it; piston in caliper presses against pad; pad rubs against disk. Servo on master cylinder boosts pressure between pedal and piston in master.
In theory, nothing in the system will move unless you put pressure on the pedal; so the brakes CANT apply themselves, they can only really come on, if the peddal is pressed.
If then the brakes are 'on' when you dont have your foot on the pedal, then the cause is mst likely to be that something is sticking or jamming, after you have taken your foot off the pedal, so that the pressure isn't released at the caliper.
Starting at the caliper end, with the 'usual suspects'.
The pads slide on guide pins through the top of the calipers and pads; there is normally a light spring between the pads to help keep them seated and stop them rattling; the ends of the pads sut in guide rebates in the caliper body.
Calipers are down behind the wheel, exposed to the elements, and get covered in crud, and corrosion; those guide pins and groves often get clogged with crap, a lot of which is 'brake dust' the black sooty like substance made when the surface of the pads wears against the disk; this, when wet is notoriousely acidic, and catalyses corrosion, which is one of the reasons that alloys if not regularly cleaned go furry, especially on the inside.
So; first check, are the pads tight? Are they free to move in their guides, or are they jamming any where. If they are looser one end that the other, as they twist one edge can actually be pulled intowards the disk, and rub.
Next; the pistons that press on the pads, are similarly exposed to all that crud. They are chrome-plated, and have a pair of rubber seals around them. When you press the brake pedal, you push the piston out of the cylinder; when you take your foot off the pedal, a spring returns the pedal, but the piston is not sucked ALL the way back in by the fluid returning in the pipe. The main thing applying the return force is the wiper seal that has been twisted a bit as the piston went out. Brakes rely on the seal applying the return force over a small distance to set the runnig clerance between pad and disk, so that the brakes self adjust.
Basically, doesn't matter how dar out you push the piston, it will only return the half mm or so that the seal returns it; just enough to take pressure off the pad; any 'gap' this creates in the hydraulic line should be filled with new fluid sucked in from the reservoir.
BIG cause of brake binding; is that the exposed pistons, that are a close tolerence fit in the cylinder, get crudded up and corroded. For a long time a large portion of the piston sits outside the seal, taking up the creance as the pads wear; then, most often, when the pads wear out completely, new pads are fitted and the pistons have to be pushed back into the cylinders; Mechanic will press the pistons back into the cylinders with a clamp or more often a big screwdriver, and the brakes will work; only being gummed up, they stick a bit in the seals and dont go back as far as the seal wants to pull them; and bit by bit the running tolerance gets smaller and smaller.
at some point, the pistons get to a point where they are so gummed up that when the pedal pressure is removed they dont move back into the cylinder at all, and they remain against the disk, just with very little pressure on them, but, each time you work the brakes it gets worse and worse, until the 'stick' on the piston means that they are putting effective braking force on the pads, and the caliper is 'binding'.... basically the piston is seized or semi siezed in the cylinder.
In bad cases, the corrosion and flaked chrome on the piston 'cut' the piston seal, and they will start to weal fluid, and need seriouse attension and overhaul.
BUT!
Those would be my starting points' looking at the condition of the caliper and pads, to see that they were free and loose and able to move in thier guides, and at the state of the pistons behind them to see if they are corroded or pitted.
It's not the brakes 'putting themselves on', but it is them 'not taking themselves off'.
Usually effects one axle of one side more than the other; as the fronts tend to get more hard wear than the backs, and the near side wheels usually get more crud than the offisde.
So check all four wheels; becouse even though the rates are differnt at each corner, corrosion is the main culprit, and if one corner has been exposed to the elements to get that bad, then chances are the others wont be far behind, even if they aren't so bad.
If the calipers are in a reasonably good state, then have to start looking elsewhere; and the next 'usual suspect' of binding is the master cylinder; depends on the design, but occassionally a failure of one of the seals in the cylinder can cause the master to 'over fill' the line with fluid from the reservour, in effect, tightening up the clerance at the caliper each time you brake, by 'over adjusting', which has the same effect as sticking pistons.
Lastly; and this is highly speculative; if there is a fualt in the master cylinder or the pedal linkage, that is making the piston 'stick', its not entirely unheard of for the pedal pressure to be released, but the mechanism to stay beyond the travel where the servo 'assists', and the servo maintains brake pressure, even though the pedal pressure has been taken off.
I have heard of this actually happening only once; but co-incidentally it was on a commercial diesil; which had a vacuum pump to provide servo suction, unlike a petrol that takes assistance pressure from inlet manifold vacuum.
And the anomoly here is; in a petrol, when you brake you take your foot off the accelerator, causing large manifold vacuum, that assists braking. Soon as you accelerate again, that vaccuum is released by opening the throttle butterfly, and if there was any residual servo assistance, it would have no pressure to sustain it.
On a diesil, with engine driven vac-pump..... it will give vaccuum s long as the engine is running; so applying throttle wont effectively switch it off......
But that is about the only scenario that I can envissage that might give impression of the brakes 'applying themselves', if the servo was sticking, you'd brake, release brakes, brake pressure would ease, but not fully release, then when you applied throttle, you would increase revs on the vac pupm, making more vacuum, which could suck the brakes on harder, making them feel like they applied themselves.
BUT, it is such an unlikely scenario, as to be almost not worth considering; Look at the 'usual suspects' first, becouse I suspect that it is a simple case of binding, rather than a poltegiest braker!
What'goodies' has the car got on it? Is it ABS, does it have TRaction Control or Hill Decent Control of anything like that on it, that work through the brake system?
Brakes wont generally try and apply themselves; unless there is a leccy brain and pump tthat might try and do that for TC. If there's leccy intervention...... brings in a whole remit of pottential causes to look at.
So, conventional system, without any microchips in it; not being patronising; but going back to 1st principles;
Pedal, end of pedal a cylinder, pipe from cylinder to caliper with another cylinder in it; piston in caliper presses against pad; pad rubs against disk. Servo on master cylinder boosts pressure between pedal and piston in master.
In theory, nothing in the system will move unless you put pressure on the pedal; so the brakes CANT apply themselves, they can only really come on, if the peddal is pressed.
If then the brakes are 'on' when you dont have your foot on the pedal, then the cause is mst likely to be that something is sticking or jamming, after you have taken your foot off the pedal, so that the pressure isn't released at the caliper.
Starting at the caliper end, with the 'usual suspects'.
The pads slide on guide pins through the top of the calipers and pads; there is normally a light spring between the pads to help keep them seated and stop them rattling; the ends of the pads sut in guide rebates in the caliper body.
Calipers are down behind the wheel, exposed to the elements, and get covered in crud, and corrosion; those guide pins and groves often get clogged with crap, a lot of which is 'brake dust' the black sooty like substance made when the surface of the pads wears against the disk; this, when wet is notoriousely acidic, and catalyses corrosion, which is one of the reasons that alloys if not regularly cleaned go furry, especially on the inside.
So; first check, are the pads tight? Are they free to move in their guides, or are they jamming any where. If they are looser one end that the other, as they twist one edge can actually be pulled intowards the disk, and rub.
Next; the pistons that press on the pads, are similarly exposed to all that crud. They are chrome-plated, and have a pair of rubber seals around them. When you press the brake pedal, you push the piston out of the cylinder; when you take your foot off the pedal, a spring returns the pedal, but the piston is not sucked ALL the way back in by the fluid returning in the pipe. The main thing applying the return force is the wiper seal that has been twisted a bit as the piston went out. Brakes rely on the seal applying the return force over a small distance to set the runnig clerance between pad and disk, so that the brakes self adjust.
Basically, doesn't matter how dar out you push the piston, it will only return the half mm or so that the seal returns it; just enough to take pressure off the pad; any 'gap' this creates in the hydraulic line should be filled with new fluid sucked in from the reservoir.
BIG cause of brake binding; is that the exposed pistons, that are a close tolerence fit in the cylinder, get crudded up and corroded. For a long time a large portion of the piston sits outside the seal, taking up the creance as the pads wear; then, most often, when the pads wear out completely, new pads are fitted and the pistons have to be pushed back into the cylinders; Mechanic will press the pistons back into the cylinders with a clamp or more often a big screwdriver, and the brakes will work; only being gummed up, they stick a bit in the seals and dont go back as far as the seal wants to pull them; and bit by bit the running tolerance gets smaller and smaller.
at some point, the pistons get to a point where they are so gummed up that when the pedal pressure is removed they dont move back into the cylinder at all, and they remain against the disk, just with very little pressure on them, but, each time you work the brakes it gets worse and worse, until the 'stick' on the piston means that they are putting effective braking force on the pads, and the caliper is 'binding'.... basically the piston is seized or semi siezed in the cylinder.
In bad cases, the corrosion and flaked chrome on the piston 'cut' the piston seal, and they will start to weal fluid, and need seriouse attension and overhaul.
BUT!
Those would be my starting points' looking at the condition of the caliper and pads, to see that they were free and loose and able to move in thier guides, and at the state of the pistons behind them to see if they are corroded or pitted.
It's not the brakes 'putting themselves on', but it is them 'not taking themselves off'.
Usually effects one axle of one side more than the other; as the fronts tend to get more hard wear than the backs, and the near side wheels usually get more crud than the offisde.
So check all four wheels; becouse even though the rates are differnt at each corner, corrosion is the main culprit, and if one corner has been exposed to the elements to get that bad, then chances are the others wont be far behind, even if they aren't so bad.
If the calipers are in a reasonably good state, then have to start looking elsewhere; and the next 'usual suspect' of binding is the master cylinder; depends on the design, but occassionally a failure of one of the seals in the cylinder can cause the master to 'over fill' the line with fluid from the reservour, in effect, tightening up the clerance at the caliper each time you brake, by 'over adjusting', which has the same effect as sticking pistons.
Lastly; and this is highly speculative; if there is a fualt in the master cylinder or the pedal linkage, that is making the piston 'stick', its not entirely unheard of for the pedal pressure to be released, but the mechanism to stay beyond the travel where the servo 'assists', and the servo maintains brake pressure, even though the pedal pressure has been taken off.
I have heard of this actually happening only once; but co-incidentally it was on a commercial diesil; which had a vacuum pump to provide servo suction, unlike a petrol that takes assistance pressure from inlet manifold vacuum.
And the anomoly here is; in a petrol, when you brake you take your foot off the accelerator, causing large manifold vacuum, that assists braking. Soon as you accelerate again, that vaccuum is released by opening the throttle butterfly, and if there was any residual servo assistance, it would have no pressure to sustain it.
On a diesil, with engine driven vac-pump..... it will give vaccuum s long as the engine is running; so applying throttle wont effectively switch it off......
But that is about the only scenario that I can envissage that might give impression of the brakes 'applying themselves', if the servo was sticking, you'd brake, release brakes, brake pressure would ease, but not fully release, then when you applied throttle, you would increase revs on the vac pupm, making more vacuum, which could suck the brakes on harder, making them feel like they applied themselves.
BUT, it is such an unlikely scenario, as to be almost not worth considering; Look at the 'usual suspects' first, becouse I suspect that it is a simple case of binding, rather than a poltegiest braker!
alfie2.5hse
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Teflon, why are you so vague with your response!!!!!!!!!!!!!!
slob
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are you sure its the brakes and not just you over heating?
Trewey
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ffs teflon, is your real name Tolstoy?
Teflon
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Look people; some-one asks a question, I'll try, if I can to answer it.
that's what message boards are about.... or at least I thought they were
some people seem to have different ideas.... if that's what gets your rocks off..... bit like the kids at the back of the class pulling faces and flicking stuff of rulers.
I dont GaS!
If you dont like my posts, you dont like the length of them... simple answer; don't read'em.
that's what message boards are about.... or at least I thought they were
some people seem to have different ideas.... if that's what gets your rocks off..... bit like the kids at the back of the class pulling faces and flicking stuff of rulers.
I dont GaS!
If you dont like my posts, you dont like the length of them... simple answer; don't read'em.
yella
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he's lonely
yella
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ooh, looks like teflon aint so non stick after all
slob
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ffs laddo,, if we dint read them then we wint know if we like them or not
chabrolly
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Hi Mate !
You Must Check The Hoses Perhaps One Of The System Is Blocked ...that Cause The Brakes Not To Work Properly ...or Somtimes To Be Under Selfpressure ...
You Must Check The Hoses Perhaps One Of The System Is Blocked ...that Cause The Brakes Not To Work Properly ...or Somtimes To Be Under Selfpressure ...
