Dual circuit brake failure ?

[tvrjohn]

Hi
recently the brake pedal has dropped and only works from half way
not spongy pedal

was thinking that maybe a brake pipe had burst

jacked the vehicle up and i can rotate either rear wheels when the brake pedal is fully pressed with the engine running !

looked all around the rear hydraulics, and there are no visible signs of brake fluid anywhere ?

checked the resevoir and front and rear sections are full of fluid as normal

What do you think

master cylinder seal failure on rear set ?

 

[Mursal]

If you left the wheels dangling, the suspension position sensing valve (if one is fitted) cut the pressure to the back wheels.
Or the rear wheels cylinders are seized/leaking. Might be the center seal in the master cylinder, but check the back first. If you pull the handbrake does it change the position of the brake pedal. If so. automatic rear wheel brake adjusters (if drums) are not working. They rarely work properly for any length of time.

 

[tvrjohn]

Thanks for your comments
however i'm sure i've checked the rear brakes when jacked up previuosly and they worked then.

Discs at rear, with seperate mechanism for handbrake, so operation of handbrake would not affect pedal on this vehicle.

Can't see how both rears would seize at the same time ?

any other ideas ?

 

[Mursal]

Check for pressure at the back calipers and let us know the outcome

 

[Mursal]

Duplicate post

 

[tvrjohn]

Ok jacked the rear up and removed both wheels, bled the rear calipers.
No air bubbles but rear brakes now work, but pedal remains only functional half way down ?

any ideas

 

[Mursal]

After pumping the brake pedal with the engine stopped, does the pedal become easier to push when you start the engine and the vacuum servo kicks in?

Obviously you have checked this, just curious to know the outcome of the test?

 

[tvrjohn]

Servo vac system seems ok

what are your thoughts

 

[Mursal]

Do a full system bleed, and check for flexible pipes about to fail. See if there is any history of ABS module failure/recall

 

[jctilingservices]

i am probably way off here, but are your rear brakes adjusted correctly, obviously if there is a distance to travel before brakes connect with drums / discs it will affect pedal tarvel, well thats my thoughts and i am probaably wrong. or try claamping ya flexi pies and releasing each in turn while checking brake pedal for travel

 

[Avocet]

If the braking system is a front-rear split, then yes, I guess that could be a failed circuit (the rears) in the master cylinder. More commonly these days, they're diagonally split, in which case, you'd be able to turn one front and the diagonally opposite rear. Are the pipes from the master cylinder positioned such that you can easily swap them over? If the problem then moves to the fronts, that would be pretty conclusive.

 

[Mursal]

On a good system, just keep in mind that the front do about 80% of the braking and the rear 20%, so it can be easy to say the back ain't working.

 

[tvrjohn]

Okay guys, after bleeding the rear calipers the pedal is up and rear brakes work now, Strange but now working

Thanks for your comments