Disco 1 1998 300tdi - failed MOT rear brake pressure

Crunchy · Dec 13, 2018

Just failed - tester thinks it may be compensator valve (?) - failed due to lacking in pressure at rears only.

Can anyone enlighten me on this or is there another likely culprit?

No signs of pressure loss, pedal firm but sinks on engine start up. Pads all good.

Where to start please - rear calipers, flexy hoses, master cylinder? Does it have a compensator or brake bias valve that could be causing lack of pressure to the rear?

Thanks guys!

dieseldog69 · Dec 13, 2018

Brake bias valve located on drivers side inner wing.

Both calipers would have to be sticking at same time and the flexy pipe from the boot floor cross member down to the back axle seldom gives any troubles unless badly corroded or torn off when off roaded.

Should take an hour tops including bleeding brakes.

jamesmartin · Dec 13, 2018

he cant measure the pressure just the braking action, maybe caliper pistons are seized , pads baked hard and poor discs

Crunchy · Dec 13, 2018

Cheers chaps - no-one to push pedal at present but had a good visual - all pipework looks good.

Two circuits from MS run to Loockheed valve on NS inner wing

- then two circuits to front NS/OS and rear single pipe to T junction on rear diff. Only small section of flexy pipe runs from floor support member to T. All looks good.

Will whip off wheels and see how calipers/pistons/pads look?

If all is working smooth what's next?

Crunchy · Dec 13, 2018

Update - had both sets of pads out - I put in some thin packers and pressed pedal, then levered back the pistons (repeated again) = both sides and pairs of pistons are moving smoothly all the way in out - pads looked evenly worn, maybe slight glazing so I have cleaned/roughened that off. Discs look good, no ridge to speak of, clean/even wear.

And I got someone to stand on the brake pedal - again all good, flexible pipe from rear floor bracket to 'T' is fine it seems.

Crunchy · Dec 16, 2018

Hi - anyone have any more thoughts?

mr glee · Dec 16, 2018

master cylinders run a duel circuit one for front and back (I have had this before not on a disco) maybe the seals have worn not allowing the pressure to build pedal will still go firm but will travel a bit further than usual.

mr glee · Dec 16, 2018

Just a thought air in the master cylinder have you had the brake lines open to the point it ran dry at any time as this would be a different bleed process to get the air out.

Crunchy · Dec 16, 2018

Thanks, that sounds very possible.

I guess if the servo, pipes, calipers, pads and discs all appear to be OK then all that is left is the bias valve and master cylinder.

I don't want to waste time and money obviously changing both if unnecessary - is it possible to check which may be faulty - the bias valve is letting fluid to the rear but not sure if it changes behavior under hard braking or how to check this?

Crunchy · Dec 16, 2018

mr glee said:
Just a thought air in the master cylinder have you had the brake lines open to the point it ran dry at any time as this would be a different bleed process to get the air out.

Not in my ownership (three years).

mr glee · Dec 16, 2018

If you're able to make brake lines you can give the brakes a direct feed from the master cylinder also make the front feed from the master cylinder go to the rear brakes that what I did to isolate the problem.

Crunchy · Dec 16, 2018

Ahh, a bit beyond me sadly!

Rodeo Joe · Dec 16, 2018

Sounds like they just need bleeding.

mr glee · Dec 16, 2018

Bias valves normally either start to leak or allow the rears to lock up early when they fail not sure if they can fail in the way your brakes have.i would go for the master cylinder so long as your sure everything else is ok no kinks or crushed brake lines?.

Rodeo Joe · Dec 16, 2018

If you've had it 3 years and done nothing to bleed the brakes start there before you start tearing it to bits.

discool · Dec 16, 2018

If you think there’s air in the system then bleed the brakes, but after three years it’s better to also replace the fluid as well.

Crunchy · Dec 16, 2018

Thanks chaps - understood - will bleed through the whole system with fresh fluid before replacing anything.

It's never lost any fluid and everything that should be dry is dry - all pipework appears A1 visually and the one flexy pipe also appears fine (and under pressure).

Rodeo Joe · Dec 17, 2018

If the fluids old it can absorb moisture .

Crunchy · Dec 18, 2018

I pressure bled through with new fluid this morning and that and the bit of fiddling with the calipers and pads and wahey! I got a pass. Not a huge amount better but enough now to satisfy the examiner.

The Disco lives on more borrowed time but at least I've got another year with the old girl.

Thanks all for the advice and help.

Disco 1 1998 300tdi - failed MOT rear brake pressure

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