Brake Troubleshooting - looking for help

Hi all,
I hope I can draw on the hive mind's experience with this one before it costs me a fortune to fix.

Car is a 1996 300TDi 110

Brakes on this were great and could stop big heavy trailers, lock up the wheels etc. no problem until last week.

On the way back from the Dordogne, as we got to Normandy for a few days camping, the brake pedal went very stiff and it became hard to stop, needing almost double-footed effort to stop.

The vacuum pump was replaced a few months ago, but I popped the vac hose off and tested it with my finger while the engine was running and it still sucked my finger onto the pipe.

Some googling of various forums on my phone pointed towards the problem being the booster. Ordered a new one from Bearmach and fitted it (including the new o-ring between Booster and Master Cylinder).

Now there is a different problem - lack of braking force. Whereas a month ago the car would stop fine, then a week ago the pedal was stiff all the way down, now there is very little braking force at all and it's really hard to stop the car. A test of an emergency stop from 40mph on wet tarmac (which would previously have locked the wheels) required far more room to finally stop and was quite scary in the lack of braking force.

Fluid level in the reservoir seems the same as before, I did not open up the hydraulic system when I changed the booster.

Here's the odd thing - when braking in reverse, the stopping power and brake effort seems much more similar to how it was originally (ie. good).

The amount of brake pedal travel to make the brake system start working is much more than before, and it feels spongy.

I've read a lot of threads on this but not found many people who actually had the same sounding problem and went on to solve it.

I have tried to do this test:
1. With engine off, pump brake pedal until it goes hard
2. Start engine with brake pedal held down to see if pedal moves.

BUT the brake pedal just does not go hard at all, nothing seems to change.

My mechanic friend suggested the master cylinder could be at fault.

No evidence of brake fluid leaks in the pipes as far as I can tell.

Has anyone had experience of this?

Many thanks

The symptoms do sound like the servo (booster) isn't working for whatever reason. If your pedal does not dip during the test procedure then your servo (booster) is not working.
When you test the servo pumping the pedal with the engine off is supposed to clear the vacuum from the system and when you start the engine with the brake pedal depressed you should feel a distinct dip from the pedal as the vacuum gives it a shove.
You know that your vacuum pump is good as it is new and you have tested it. (Why did you replace the vacuum pump?)
The servo is new, so should be OK, but from what you say it is not working.
I would have a close look at the pipe from the vacuum pump to the servo for any damage or blockage not forgetting the non-return valve .
Did the new servo come with a new non-return valve?

I would whip the wheels off and check your brake pads/shoes/slave cylinder seals as well. And check flexi pipes for any bulging under brake pressure.

If it passes the servo test then I would suspect the seals in the master are on the way out.

Firstly, thank you all for your replies, very helpful.

Rougharse Racing said:

The symptoms do sound like the servo (booster) isn't working for whatever reason. If your pedal does not dip during the test procedure then your servo (booster) is not working.
When you test the servo pumping the pedal with the engine off is supposed to clear the vacuum from the system and when you start the engine with the brake pedal depressed you should feel a distinct dip from the pedal as the vacuum gives it a shove.
You know that your vacuum pump is good as it is new and you have tested it. (Why did you replace the vacuum pump?)
The servo is new, so should be OK, but from what you say it is not working.
I would have a close look at the pipe from the vacuum pump to the servo for any damage or blockage not forgetting the non-return valve .
Did the new servo come with a new non-return valve?

Click to expand...

Hello RR, the vacuum pump was replaced as the old one rattled a lot and was on the way out.
The non-return valve currently fitted is the new one that came with the booster. The pipe is quite old from the look of it, so possibly a culprit.

Scooby22 said:

I would whip the wheels off and check your brake pads/shoes/slave cylinder seals as well. And check flexi pipes for any bulging under brake pressure.

Click to expand...

Thanks Scooby, I have read about these and these are on the list if I can exhaust all options around the booster. What I can't get out of my head is 'what is the likelihood these components failed at exactly the same time as the old brake booster?' As changing the booster has at least gotten rid of the solid pedal problem, pointing toward the old one being faulty.

tottot said:

If it passes the servo test then I would suspect the seals in the master are on the way out.

Click to expand...

Thanks Tottot. Something worth checking next then (as above) - just want to make sure I'm not unnecessarily changing parts.

What I've done today (on the advice of another LR forum member) was to adjust the push rod inside the booster. I removed the unit, then the push rod has an 8mm hex head on the stem of the push rod, and a dome headed 6mm nut on the end. I was able to get a spanner on the stem, hold it with pliers, and wind out the dome headed nut by about 10mm (did initially adjust it to 5mm as the original had a 5mm gap).
What this has done is raise the biting point of the brake pedal up to near where it was before the first booster went kaput (after fitting the second booster the biting point was much further down than before). I now have a lot more braking force with less pedal effort than when the old booster was broken. I am still not certain if this has fixed the issue, or masked the problem, because.....
Did the servo test. Pumped brake pedal which felt stiffer (though not rock hard). Started engine, thought I could feel movement but maybe I was subconsciously pushing harder.
Emergency braking test from 40mph much better as I am able to exert more brake force.
Started to pour down so packed up for today. More testing required I think.

The pedal does not have to go rock hard with the servo test, it just gets harder as there is no assistance. The important bit is that the pedal dips slightly when you start the engine.

Thanks for the clarification, tested again this evening, the servo test does not make the pedal dip when I start the engine, so I suspect all I have done is to move the bite point.

Moving the car back and forth and applying the brake, it doesn’t feel like it is ‘biting’ down on the discs properly. It’s quite easy to overpower the brakes with the clutch.

Might try swapping the non return valve for the old one.

If this booster is faulty that’ll be annoying, I hope Bearmach would accept the return as I’ve scratched it to buggery already.

Have you checked the measurements on the adjustment rod. Sounds to me like the adjustment rod might have been altered by mistake meaning the pedal has to move more before the master cylinder has moved.

I know you didn’t change the master cylinder but the servo could be slightly different spec to the OEM ones so the actuator rod might need adjustment.

To get the clearance spot on on line I used blue tack to measure how much clearance it had.

Google ‘brake servo rod adjustment defender’ or some such or look in the Haynes/LR workshop manual

Wait ignore my entire post lol I just skim read the above and missed that you had already done this :/

Servo sounds suspect in that case, although without servo assistance the brakes will still work just be very very heavy.

Does the brake pedal ‘pump up’ with the engine off?

Did you disconnect any hydraulic lines by the way?

Hi Flat,

Yep adjusted the rod as mentioned.
The car will stop eventually, it's just not the same as it was a few weeks ago. It feels like the pads aren't clamping down enough.
The pedal can be pumped with the engine off and it doesn't get harder at all, feels exactly the same no matter how much it's pumped.
I didn't open the hydraulic system at all, only opened the lid to have a look in before screwing it back on.

Don't forget to check that you are getting good vacuum at the servo end of the vacuum pipe to rule out the vacuum pipe failing.
It is very difficult to diagnose a fault from afar, but I would be surprised if the new servo was faulty, the vacuum pipe is the only component that hasn't been changed since the fault occurred, is it the original pipe or has it been replaced previously? If it is not proper vacuum pipe it could be collapsing under load as most normal pipes are designed to be under pressure.

Rougharse Racing said:

Don't forget to check that you are getting good vacuum at the servo end of the vacuum pipe to rule out the vacuum pipe failing.
It is very difficult to diagnose a fault from afar, but I would be surprised if the new servo was faulty, the vacuum pipe is the only component that hasn't been changed since the fault occurred, is it the original pipe or has it been replaced previously? If it is not proper vacuum pipe it could be collapsing under load as most normal pipes are designed to be under pressure.

Click to expand...

Well you were right in that the vacuum was not enough. I ended up giving the landy garage a call and went there after work yesterday. I had not tested a healthy vacuum so I did not know what I was meant to be comparing it to. When he tested the vacuum on the pipe, there was not enough, which points to the (new) vacuum pump (or the cam, which is more expensive so I hope it's actually the pump that's failed). Going to take it in on Friday to get a new pump fitted and to check the cam.

EDIT: The vac pipe was actually a hard line for most of its length, and it was checked to see if it was sucking, but no leaks.

Update is with what happens,

It not inconceivable that the master cylinder seals have failed or another fault has cropped up at the same time.

See how it is after new vac pump though

Vac pump test. Remove from servo end and with engine at idle it should stick to your thumb. If it does not it is duff.
300 Vac pump fail is common.

tottot said:

Vac pump test. Remove from servo end and with engine at idle it should stick to your thumb. If it does not it is duff.
300 Vac pump fail is common.

Click to expand...

Bonza, so basically if I put my finger over the end it would suck my tender flesh in, but not enough to stick the pipe to my finger, so points to this being the issue. Bit annoying really as while the old vac pump was rattly as heck, at least it made vacuum!

It's a basic test but usualy gives the game away.

Right, here's the end of the story.
It was the brake vacuum pump that was broken, only been fitted 2 months but goes to show I should not have discounted its failure. Wasn't even a Br***art component!
New pump fitted and we are back to good brakes, although I do need to adjust the push rod again to move the bite point to a better position.
A bit of a relief! Thanks to all of you for your help.

Good you are sorted.