Pulsing Brakes MOT failure?

tedted

Strider590 wrote: »

When the calliper pistons seize, the brake fluid doesn't return to the master cylinder as it would normally, the weight of the brake pedal causes the pedal to rest on the brake light switch and as a result the lights can get stuck on.

r
this is rubbish

Stigy

Strider590 wrote: »

When the calliper pistons seize, the brake fluid doesn't return to the master cylinder as it would normally, the weight of the brake pedal causes the pedal to rest on the brake light switch and as a result the lights can get stuck on.

That statement sure has confused me

The brake pedal doesn't interact with the hydraulics in that way. The nearest it gets is the servo....

Strider590

tedted wrote: »

r
this is rubbish

Oh really......

Less that one year ago I replaced two seized brake callipers on one of my cars, it did not fail an MOT for seizing callipers (because it's not an MOT fail), it failed because the seized callipers had caused the pedal to fall on to the brake switch, causing the brake lights to show constantly.

I could get the brake lights to turn off by using a brake calliper opening tool to force the pistons back, thus forcing fluid back up into the master cylinder.

In the end I fitted new callipers, flushed through with new brake fluid and no more problem.

AdrianC

Strider590 wrote: »

Oh really......

Yes, really.

Less that one year ago I replaced two seized brake callipers on one of my cars, it did not fail an MOT for seizing callipers (because it's not an MOT fail), it failed because the seized callipers had caused the pedal to fall on to the brake switch, causing the brake lights to show constantly.

Then you have other problems. The pedal should be returned to the stop with a strong spring, and there should be a small amount of free play before the pedal actually operates the master.

Strider590

AdrianC wrote: »

Yes, really.



Then you have other problems. The pedal should be returned to the stop with a strong spring, and there should be a small amount of free play before the pedal actually operates the master.

The return force is the hydraulic force from the master cylinder and if the master cylinder doesn't return to it's normal position because of seized callipers, the pedal will be pulled down........

I don't know why this is so hard to understand, it's some pretty basic hydraulic principles. Unless somebody invented stretchy springy brake fluid......

AdrianC

Strider590 wrote: »

The return force is the hydraulic force from the master cylinder and if the master cylinder doesn't return to it's normal position because of seized callipers, the pedal will be pulled down........

And how will it be "pulled down" against the strong pedal return spring, when the pedal isn't actually connected to the master, just pushing against it?

I don't know why this is so hard to understand

Probably because on every single vehicle that I've had the master cylinder apart on, I know that the pushrod physically can't pull the pedal back against the spring.

tedted

you could disconnect the brake pipe,push the pedal down and it will still return back up

Strider590

^^ After a certain point yes, but that depends entirely on the master cylinder. They're not all the same, there is usually a little slack before the ports close off the fluid reservoir and more slack again before the master cylinders own return spring kicks in. It's this point where seized callipers can hold the pedal down JUST enough to put the lights on, we're probably talking less than 1mm of master cylinder piston travel.

Anyhoo, I don't give a damn what anyone thinks they know, if you need more information see Google.

colino

Strider, your rubbish really drags you down towards darkmatter territory. I especially love your dismissal of the brake pedal return spring. Is yours a progressive, smart one?

Strider590

colino wrote: »

Strider, your rubbish really drags you down towards darkmatter territory. I especially love your dismissal of the brake pedal return spring. Is yours a progressive, smart one?

Any return spring on the pedal is entirely irrelevant if it's being pulled back by the master cylinder............... If the calliper is seized, then the master cylinder CANNOT return to it normal position.

The only way the pedal is going to return is if it's not actually attached to the piston from the master cylinder.
If it was attached and you tried to force the pedal up, you'd probably flip the seals in the master cylinder.

I assume everyone here understands why the brake pedal drops slightly when you turn on the engine, versus the car not running??
If the vacuum assist can do that, then the master cylinder can certainly do the same.