Help please Defender 300tdi soft brake pedal

[Fixanything]

I have a soft brake pedal:

Has been pressure bled and every other way you can imagine, almost a gallon of BF been through the pipes.
No bearing problems.
Good vacuum.
New servo a year ago, one way valve good.
New master cylinder a year ago.
New pads, loads of pad material.
Callipers all working checked when pads changed.

Again, very soft spongy pedal, almost goes to the floor before feeling any real braking, poor brake performance, rear brakes will not lock up even on loose gravel.

Any help appreciated, as it has just failed mot !!!!!

Many thanks
Mark

[Bellini]

Porus / corroded brake lines?

Sounds similar to what happened to my 300Tdi a few years ago. Turned out to be a corroded brake line.

[Fixanything]

Thanks for the response However:

Not loosing any fluid at all.
Good flow of fluid through all bleed nipples when bleeding system.

Thanks again

Mark

[LandRoverManiac]

Fixanything said:

I have a soft brake pedal:

Has been pressure bled and every other way you can imagine, almost a gallon of BF been through the pipes.
No bearing problems.
Good vacuum.
New servo a year ago, one way valve good.
New master cylinder a year ago.
New pads, loads of pad material.
Callipers all working checked when pads changed.

Again, very soft spongy pedal, almost goes to the floor before feeling any real braking, poor brake performance, rear brakes will not lock up even on loose gravel.

Any help appreciated, as it has just failed mot !!!!!

Many thanks
Mark

Is there air in the system when you bleed it? (possible leak/air ingress on seal somewhere (see above), faulty/loose bleed nipple, leaking union on one of the solid pipes/flexi pipes).

If yes, try using the bleed nipples in the reverse way - so get a syringe/some pipe and inject fluid into the system via that way. Make sure you prime the syringe so that it doesn't have any air bubbles in it. Have an assistant watching the reservoir for signs of bubbles coming up as you go round each corner in turn. Whichever side produces the most bubbles is likely to be the source/area of your air ingress. This narrowed it down for me - one corner of the vehicle seemed to have more 'air' in the system than the others when injected with fluid - that then led me to the problem.

Check that the caliper piston seals are seated correctly and doing their job - I had an issue where it was sucking air into the system when the brakes were released - yet not leaking fluid out when under pressure. Shouldn't have been possible technically - but it did (somehow). I spent weeks trying to find the source of it until I swapped caliper seals on the corner I suspected was the issue (see air bubble test above) - I was running out of ideas at that point.

Check flexi-hoses to ensure they don't balloon/expand excessively when braking force is applied. They can and do as they get older and more knackered - reducing brake force at the pads by a considerable margin. Are they originals? If so - they are probably past their best.

Does the fluid level drop after pumping the brake pedal - ergo are you losing fluid from somewhere? (physical leak - possibly a corroded line/union + letting air in).

Those are just a few random ideas that I went through on mine when it showed similar symptoms (little braking force on first pedal press - then on the second press you got a bit more). Drove me mad for six weeks while I stripped every possible part of the braking system from the master cylinder to the calipers.


I've got the opposite issue at the moment on the Disco - I'm getting braking force when I don't really want it (sticky rear calipers).

EDIT - added reply before I saw that you weren't losing fluid.

[EP3vMk3RS]

Rubber brake hoses starting to perish? Cheap to replace or upgrade to steel ones. Their compliance might increase just before they start to brake down and leak.

[paintman]

Clamp all the flexible hoses as near to the metal pipes as possible then see what the pedal does.
This will rule out the hoses & calipers as being the problem.

[Fixanything]

So far the only thing that I feel may be contributing is the rubber brake lines, has anybody changed these and fixed a soft pedal problem ? Can you physically see them expanding ?

Could it be the servo to master cylinder adjustable pin/rod incorrectly adjusted ? Or a faulty MC ?

Note: The brakes have never been great, what started the replacement of parts was a crack in the servo housing however, although a little better after replacement of the servo the brakes have never been great and have very slowly deteriorated to the point where it's now failed the mot.

The most distinctive thing is the very soft pedal that moves 3/4 of it travel before any real braking occurs, with or without the engine running.

Thank again for all your help

[camel_landy]

It's a pretty simple system.. The usual suspects have been covered but if that hasn't sorted it, maybe you need to consider you've done something wrong.

• It can be easy to leave air in the system if the bleed hose goes down instead of up.
• Double check bearing play.
• Are you losing any fluid? If not, maybe the rubber hoses are perished and bulging?
• Maybe the new master is faulty?

Try and avoid making assumptions.

Just a thought.

M

[Fixanything]

camel_landy said:

It's a pretty simple system.. The usual suspects have been covered but if that hasn't sorted it, maybe you need to consider you've done something wrong.

• It can be easy to leave air in the system if the bleed hose goes down instead of up.
• Double check bearing play.
• Are you losing any fluid? If not, maybe the rubber hoses are perished and bulging?
• Maybe the new master is faulty?

Try and avoid making assumptions.

Just a thought.

M

Could you please clarify what you mean by the "bleed hose goes down instead of up" ?

Thanks
Mark T

[paintman]

Ballooning of hoses can happen, unusual but get someone to press the pedal hard while you look at the hoses. Take the suggestion to clamp the hoses & see what the pedal does.
Just because the m/cyl was changed 12 month ago doesn't mean it isn't defective.
If you pump the pedal does it come up or stay the same? Try with engine off & vacuum exhausted. What happens?

[Fixanything]

Brake pedal is always soft no matter what conditions pumping make little to no difference, same with engine running or not.

[camel_landy]

Fixanything said:

Could you please clarify what you mean by the "bleed hose goes down instead of up" ?

When you attach your rubber hose to the bleed nipple, make sure it goes straight up so any air bubbles can rise and not get caught in the bleed nipple area.

You may well already be doing that but as were at the 'head scratching' point, it's time to ask the obvious questions too.

M

[Dave.]

Is the new brake fluid from a fresh batch/can?

[Fixanything]

Yep all fresh and about 5 ltrs has been put through the system

Thanks
Mark

[Fixanything]

Changed MC again, bled system with 4 ltrs of DOT 4, checked brake line rubber hoses all OK, this all made absolutely no differences. Took to different garage who said they could not find anything wrong with brakes, so now taking back to original mot station for free retest, if they don't pass it the other garge will ! Personally I don't think the brakes are great but not mot failure worthy ! Will continue to investigate the soft pedal.

[The Wookie]

Does it have a proportioning valve for the rear axle? If so I'd be looking into this as a possible area of system compliance, particularly as you say it wont lock the rear wheels.

Also master cylinders are a ball ache to bleed properly on the car, I'd recommend bench filling them before fitting if possible. Simply chucking more and more fluid through it wont make any difference if there's air trapped.

The short hard lines at the ends are also pretty good for trapping air, although I'd expect it to have shifted if you've been through that much fluid, assuming you're giving the pedal a good shove when bleeding.

Be suspicious of any power-bleeding or vacuum bleeding equipment, they can introduce small amount of air suspended in the fluid even when working properly, if faulty or misused then it can easily make for a cack pedal.

Other than that, flexi hoses as has been said and double check for leaks anywhere. If you continuously find air after driving the car for a while then this is your issue; the system will draw in air through leaks small enough that it could appear as a weep with minimal fluid loss.

Failing all of that then have an assistant check for flex/compliance in everything while you stand on the pedal; calipers, hoses, even the pedal and mount itself. You should expect to see a minimal amount of flex in any given part but it should be relatively obvious if something is moving excessively

Edited by The Wookie on Thursday 17th November 11:09

[tight fart]

Have you tried locking the pedal down overnight?
No idea why this works but worth a try, easy on a defender as well.

[Fixanything]

Fixanything said:

Changed MC again, bled system with 4 ltrs of DOT 4, checked brake line rubber hoses all OK, this all made absolutely no differences. Took to different garage who said they could not find anything wrong with brakes, so now taking back to original mot station for free retest, if they don't pass it the other garge will ! Personally I don't think the brakes are great but not mot failure worthy ! Will continue to investigate the soft pedal.

Passed mot 😀 ! However, not withstand the vacuum pump tested OK and both independent garages also convinced themselves that the vacuum pump was ok, we changed the vacuum pump today, guess what, big improvement, probably close to new performance, saying this, I still think the pedal travel is excessive and still can not lock rear wheels but they are braking !!! However I do not have another Defender 300tdi to compare it to, so maybe normal !

Thanks for all of your help and advice.