Disco 1 1998 300tdi - failed MOT rear brake pressure

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!

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.

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

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?

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.

Hi - anyone have any more thoughts?

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.

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.

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?

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.

Click to expand...

Not in my ownership (three years).

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.

Ahh, a bit beyond me sadly!

Sounds like they just need bleeding.

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?.

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

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.

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).

If the fluids old it can absorb moisture .

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.