Any one got any ideas why my rear disks on my mk woukdvsuddenly bind on. Came home approx 10 miles and when I got back I could smell the rear pads had been binding and the wheels were pretty warm. The brake pedal was rock hard with no travel. Fronts where fine. Checked the hand brake and it was off. Once cooled the pedal moves a little again and they are not binding but still mot right. I'm assuming as only the backs it probably the break bias gone bad now allowing the brake fluid to flow back to the master cylinder. It's a standard sierra master cylinder. Any one got any ideas before I hear things apart. Also anyone no what part the brake bias is.

Cheers
Jon

Could your rod from pedal into MC be slightly too long?

It's something new as done over 400 miles these last few weeks and has been till now so it's like something has start to fail but just not sure what.

If you're using the standard Sierra Master Cylinder what sort of device is the 'brake bias' that you mention?

Did this part also come from a Sierra?

Is it screwed into the port of the Master Cylinder?

Is it a separate device plumbed into the brake pipes somewhere?

Got any pictures?

[Edited on 1/10/18 by JAG]

I'll get a pic this evening. It's mounted in rear axle the brake line comes from the front of car into this then splits in two left and right rear brake lines. It has a plunger on top with a bolt that screws down onto it.im assuming to adjust the pressure.do not know where it's from I didn't build the car.

Like this?

Check the clearance on the push rod on the master cylinder. You need 1mm of free play between the push rod the piston, to allow the return port to open. Are you using a pedal return spring?
If that does not solve the problem you will have to strip down and inspect the rear brakes. Davew823

No I'm not using a return spring.ill take the plunger out tonight see if that helps but fairly certain that's all fine as nothing has changed.i have checked for stones etc stopping the piston fully returning. Plus the fronts are fine so the return port is opening in that. What amount the return port on the rear circuit being blocked that could be an option. The pedal fully returned but was rock solid almost as if there was too much fluid in the lines but that can not be possible can it?

What state are the rear brake calipers in?

Any chance (if they're floating) that the sliders and siezed or even that the pistions themselves are not returning as they should?

If the calipers aren't working correctly they may not be releasing pressure from the brake pads and causing them to bind against the discs.

Calipers look good iv not pulled them yet however. I doubt both could have failed at the same time.must be something to do with master cylinder or break bias unit. I'm going to release the pressure via the bleed nipple and see if the calipers back off. That will prove calipers are good

Sensible approach.

Removing the bias adjuster from the system would identify whether that is at fault as well. Would mean topping up and bleeding the brakes, but you're most likely going to have to do that at some point anyway.

Is the handbrake cable(s) & mechanism free and actually releasing at the hubs ?

I would say not enough clearance on the brake rod to the rear master cylinder

Problem solved. I'm taking the carpet put of the foot well I noticed a washed was stuck under the brake pedal stopping it returning fully. I opened the bleed nipple on the rear to allow the pedal to be pressed so I could remove the trapped washer

Always best when it's the simple things

Well done for checking before dismatling the braking system.