Brakes seized?????

[Gary Mead]

Need some help with diagnosing a brake problem,had to be recovered home today as the front brakes (n/s worse)locked up then seemed to hold on,would let go intermittently,the red dash light with the ! in the middle was flashing intermittently but is now on all the time.It seems to have lost brake fluid from the resevoir from max to just under the min line???but no signs of a leak as yet?...There was a grinding metal on metal noise and pedal seemed to be soft then jerk back under foot then be hard??
The AA man took off n/s front wheel and said pads were ok and nothing out of place but agreed that something very wrong!!!.Had wife and kids on board and was about to get on the motorway for a 130 mile trip so could of been real bad news
My Mighty CRUISER going home on a recovery truck

Im gonna have the wheels off tomorrow and have a good look for myself as im hoping it is just the pads gone thru otherwise im stumped,as usual all help greatfully received.

thanks peeps.

if it is discs pads etc wheres best places to order some new bits and poss some hoses too.

 

[Ian Rubie]

It is unusual for a disc brake to lock on, sounds more like a wheel bearing to me.

Ian

 

[Ecky Thump]

Really odd, but I went to help out a guy in a Shogun the other day. His rear n/s had been locking on and on this occasion it was his front n/s. Strange as I thought braking systems crossed over i.e. f/n/s to r/o/s

It seemed to be an issue with the master cylinder and if he braked hard he would have to stop and bleed the offending caliper until the brake literally thudded off.
Never come across this before and he decided to take it to Mitsubishi for a diagnosis.

I'm not saying Ian is wrong, just my recent experience

 

[Paul]

Had this before (not on an 80) when the brake fluid was really old as it can absorb water, so when it got hot the fluid would expand and lock brakes on.

 

[Ecky Thump]

Had this before (not on an 80) when the brake fluid was really old as it can absorb water, so when it got hot the fluid would expand and lock brakes on.

That may well explain the Shogun Paul as it was a very hard worked unserviced farm vehicle.

 

[Andrew Prince]

Ahem...(putting my pointy hat on)

Fluids (technically liquids, as gases are also classed as fluids) are incompressible - this means that their volume does not change with temperature or pressure. Hence liquid is used for brakes, not air or some other gas - you want the force to transmit to the caliper, not get absorbed by compression.

So it is not possible for the fluid to get hot and expand any appreciable amount. What is being described here is the water in the brake fluid boiling, i.e. becoming a gas, which drastically increases the volume of fluid (liquid + gas) operating in the braking system, thus increasing the pressure and locking the brakes So this explanation makes sense if the water is boiling. The fluid must be really old and have a lot of water in it to boil, even under heavy braking...

Cheers,

 

[Paul]

Is the word you are looking for Hygroscopic?
This is a fault seen on many cars and there is even a tester available with two probes or contacts that will measure the conductivity of the fluid which should represent the water content.

Phew, that takes me back to college days

 

[Andrew Prince]

Is the word you are looking for Hygroscopic?

Nope Brake fluid is hygroscopic in that it readily absorbs water, which is why brake fluid needs changing every so often. Incompressibility of fluids has nothing to do with hygroscopicity - brake fluid and engine oil, for example, are both incompressible but one is hygroscopic and the other is not :confusion-seeingstars:

I was going to include a spiel on that in my lecture above but thought better of it

 

[Paul]

Thats it, i was refering to the fluid itself.

 

[Andrew Prince]

Thinking more about the story above, it is possible even with brand new brake fluid to get it hot enough to boil (i.e. on a racing car where there is frequent, heavy braking). Boiling brake fluid would equate to the example above yet racing cars don't suddenly have locking brakes if the fluid overheats. Rather the braking performance is diminished and the pedal becomes spongy, i.e. the gas that's formed compresses and absorbs some of the braking force.

So why would water boiling in the caliper or brake lines be different to the spongy brakes and cause the brake to lock? :confusion-scratchheadyellow:

Maybe there is something more to the story than the physics theory?

 

[Paul]

Does this mean we can all go out and buy high performance brake fluid now.......

 

[BobMurphy]

A couple of observations . . .

1. The volume of a Liquid will change with temperature (but not drastically).

2. When you put new pads in the calipers you push the pistons back, that pushes the excess fluid back into the master cylinder reservoir (from where it can overflow ). Its only when the brake pedal is pushed that the return valve shuts and allows brake pressure to be applied.

Ergo; if the car is stationary and the brake pedal isn't being touched, any excess pressure from e.g. vaporised water, should be released. The brakes will be spongy when used if there is vapour in the line though.

The cause of the problem sounds like something else to me.

Bob.

 

[Dave Docwra]

More likely causes rather than fluid could be:-

Rubber brake hose internally delaminated and acting as a one way valve

Caliper sliders seized

ABS solenoid valve faulty

Master cylinder Faulty, but this would normally holds on all brakes.

 

[Jon Wildsmith]

the front brakes (n/s worse)locked up then seemed to hold on,would let go intermittently,the red dash light with the ! in the middle was flashing intermittently but is now on all the time.It seems to have lost brake fluid from the resevoir from max to just under the min line???but no signs of a leak as yet?...There was a grinding metal on metal noise and pedal seemed to be soft then jerk back under foot then be hard??

What makes you think it was the front brakes that locked up - I mean how did you determine front as opposed to the rear? The reason I ask is just that the rest of your problem description fits losing an inside brake pad on the rear ...

 

[Gary Mead]

definetly locked up on the n/s/f the screech across the car park as if i had stood on the brakes and was skidding!!
im going to get the wheels off tomorrow and have a proper look.

cheers gary

 

[Andy Harvey]

Sounds like eithean ABS pump problem or that the caliper has seized solid on at least one of the 4 pistons - not unknown if the seals have gone and a lot of water has got in. And I know the problem that Jon talks about, lost the inside pad on my offside rear when I had let it wear down to much just as I had started to go down the Llanberis pass,boy did I have to take it carefully and when I took it apart both the caliper and the disc were toast. Had to be recovered back to Cheltenham as I couldn't get the parts while I was up in Wales.

 

[Gary Mead]

It turned out to be the n/s rear brakes!!!!(so you were right Mr Wildsmith) ....the wheel was coverered in rusted metal filings,and ruined the wheel,the plate is in there but the pad has dropped off/out...easy/cheap fix so im pleased about that ..i was getting worried it could be more serious

i really thought it was the front locking up,so will keep an eye on it for a while but all should be fine now


phoned around local places for pads for prices etc,mintex,ebc various cheap ones but then a name called Pagid that every one seemed to think were pretty good.any views on these as ive not used before???

cheers Gary

 

[Andrew Prince]

Welldone on getting to the bottom of it - and to Jon for diaggnosing correctly

phoned around local places for pads for prices etc,mintex,ebc various cheap ones but then a name called Pagid that every one seemed to think were pretty good.any views on these as ive not used before???

Nothing wrong with Toy OEM ones, other than the price maybe?

 

[Jon Wildsmith]

Glad you got it sorted. Usually the whole pad falls out and drops down between the disk and the backing plate when you let them get too low leading to knackered piston & disk so that's lucky you just need pads

 

[Dave Docwra]

The devil is in the Detail