Xantia MK II - Low rear brakes efficiency

This is the Forum for all your Citroen Technical Questions, Problems or Advice.

Moderator: RichardW

lancia58
Donor 2022
Posts: 685
Joined: 30 Nov 2010, 08:02
x 6

Re: Xantia MK II - Low rear brakes efficiency

Post by lancia58 »

So is there a way to test the rear brakes before MOT ?
User avatar
Mandrake
Posts: 8542
Joined: 10 Apr 2005, 17:23
x 440

Re: Xantia MK II - Low rear brakes efficiency

Post by Mandrake »

lancia58 wrote:
09 Jul 2019, 14:04
So is there a way to test the rear brakes before MOT ?

Not really - not in a way that would give you an idea of pass/fail.

I usually just try a few hard stops from speed and judge how much the car lifts at the back. If the car lifts a lot at the back the rear brakes aren't working properly. If it only lifts slightly or doesn't lift at all they are usually working well.

When you brake the centre of mass of the car is thrown forward, this will try to compress the front suspension and extend the rear suspension, so if the rear brakes don't work this momentum will throw the rear suspension right up to the top limit stop. Also the torque from the front brakes will try to lift the rear suspension as well.

When the rear brakes work the torque from the brake clamping on the turning wheel will try to rotate the trailing suspension arm forwards, this will compress the rear suspension. So the rear brake torque works to counteract the natural rise of the rear suspension under braking. Ideally they equal each other in which case the car stays level under braking.

If the rear brakes are working well and the front brakes are not working properly you can even see the rear suspension duck under braking... :lol:

You will also see this effect when the car is on the brake tester machine - if it's only a single axle testing machine the rear suspension will squat down as the rear brakes are tested.
lancia58
Donor 2022
Posts: 685
Joined: 30 Nov 2010, 08:02
x 6

Re: Xantia MK II - Low rear brakes efficiency

Post by lancia58 »

Hi

After changing the rear spheres and bleeding the brakes it passed the MOT today. Thank you all for your help

Zohar