Disco 2 Servo problem

2003 TD5 ES Manual. 112K miles.
With engine off, the brake pedal is firm. Engine running and the pedal will slowly sink to the floor.
(This is well beyond the usual initial dip) The pedal stays firm if the engine is run with the serpentine belt removed so that the vacuum pump isn't running. Therefore I assume it's a servo issue.
Is there a way to test the pump, before I order a servo?

I'll let others to intervene cos i said it too many times that this is normal, so does mine too and it has a perfect braking distance... only one thing, since you fitted the TRW cylinder and you have firm pedal with engine off did you drive test it to see how is it braking? Cos if it stops well you'll spend money in vain on a new booster as the servo is supposed to keep pressure in the system even with the pedal at the floor as long as it doesnt have a leak but that would cause a hard pedal not sinking pedal, booster failure will eventually increase the braking distance not the pedal's travel. Good luck, i'm watching this with interest cos you'll be the first i see with this symptom.

I've driven this Disco 22K and the braking changed about a month ago when this started. I've been using it today and while they still feel safe, the brakes are not as sharp as they used to be, with a softer pedal. It is also disconcerting to be stopped at lights and feel your foot slowly sinking! (Still very firm with engine off)
I will definitely replace the servo (over £300) as soon as I can find one. Like good-make master cylinders, servos seem to be difficult to find. Even the German supplier of the TRW master cylinder are out of stock. There don't even seem to be any service kits available any more. I've rebuilt loads in the past for cars I've restored - MGA, MGBs, MGC and many others.
I changed the servo (which had no obvious fault) on my Disco 300 and the effect was dramatic. Even my MOT guy asked how I got the pedal so firm!

I can only assume you have installed a vacuum in line from the vacum pump to the servo, to confirm the vacuum pump in OK, and that the servo is not losing vacuum until you press the brake pedal
and that before you spend £300 + you have confirmed that the servo of vacuum pump are in fact faulty, another way to check your breks is to visit a local MOT centre and ask the to check them on the rolling road

Before you spend any cash, check the system, take the wheel off inspect the calipers, make sure they are in good condition and not seized, and the pads, are moving equally on the disc, check all 6 of you flexi, hoses make sure they are not ballooning and need replacing, carry out an old fashioned brake bleed, before you spend any cash remove any doubt find out to the best you can what the problem is

Thanks frostythor, Delphi pads have done 250 miles. Caliper sliders were all greased when pads fitted. Hoses all seem fine. With engine off, the pedal is so firm that I can't believe there is air present.
Braking isn't too great - it is just about possible to engage the ABS with a sharp jab on the pedal. I know the car well and it should stop much better than this.
I haven't measured the vacuum pressure, as the way the pedal feel changes with the engine running ,made me assume that pump & feed pipe must be OK
Regarding the servo, Rave just says disconnect the pipe, but omits to say how! I don't want to knacker the rubber connector! Does it just pull out or is it a twist & pull?
The car is a keeper, so am prepared to spend what it takes to fix it, but definitely can't afford to waste money!

the servo pipe does have 2 ends, if you cant get it out of the servo, take the other end off, and as sierryfery say's the servo is there to increase the brake fluid presure at the pedal by assisting you without it a lot of people would not have the leg power to brake

Tim C said:

Braking isn't too great - it is just about possible to engage the ABS with a sharp jab on the pedal.

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Are you sure it's ABS and not EBD ? the feedback is pretty the same for both just that for EBD the pedal gets stiffer and slightly pulsating and kicks in at lower speeds while on the ABS it's pulsating harder and doesnt get so stiff but kicks in from higher speed, not easy to identify them but if it's a servo problem it should rather be EBD cos it acts on axle pairs not on the wheels individually and if the servo doesnt deliver enough power to the master cylinder that would affect the deceleration rate of all the wheels not just one or two(provided there's no air in the system)... very complicated issue you have there... what dimension tyres do you have? did you by any change fit bigger tyres before this saga has begun?

As I see it if you pedal is rock solid, with the engine off then, the pedal goes down when it running the vacum pump is pulling a vacum on the servo so it apears to be working fine, if the pedal did not go down then I would say its not working

Tim, this is the second post you have done with your brake problem, and I can see looking back you have done a lot of work on you brakes before you had this problem so I only going to ask one question are your bleed nipples on top of the calliper or on the bottom as you can swap them over ,as we have had this on the forum before were some one put them on updside down

Sounds like EBD Sierrafery. Wheels are standard. Yes Frostythor, (brake bleeding woes) this is still the original problem which I had misdiagnosed as leaky master cyl seals. I was only able to rule out Master Cyl when Sierrafery found me a TRW one in Germany and it bled out to a hard pedal in half an hour! I started another thread as the cylinder saga was a red herring!
The calipers are correctly fitted. The truck had two owners before me and had always been serviced by the same LR garage from new. Hence it is a keeper!
Incredibly, servos now seem to be rarer than genuine master cyls, with no date for availability. Even the people who had the master cyl don't have any.
In case I don't find a new one soon, I've taken a chance and ordered a used one from a breaker for £40 delivered! (the same serial numbers)
Am still hoping to find a new one though, even at £350!
My past experience with faulty servos was that you had to shove increasingly harder to stop, but the pedal certainly didn't get soft. But then from memory, those cars had a remote servo, not behind the master cylinder and must work differently.
Certainly I have had loads of modern cars in recent years, but company cars went to the garage ...a lot as I did 70K a year! My own cars got as far as me peering under the bonnet, hastily shutting it and off to the garage! That is another reason why my D2 is a keeper!

Very interesting issue, with this type of servo behind the master cylinder the only logic i can find for your symptom is that the servo is loosing power on the output for some reason then the pressure to the cylinder is not enough and if the decelerataion rate is below expected the ECU activates the EBD, would be the first time i see such thing but we all learn while we live

it appears in stock here https://www.lrdirect.com/sjg500020-booster-brake but better wait untill you try the used one

Thanks for the link, as you say, maybe try the used one first! I haven't found one in the UK yet. I hope the issue doesn't get any more interesting and that another servo sorts it!
Rave p1054 shows a good diagram of a servo. The principle is fairly straightforward, but I can't remember the detail of how they work. The bit I really can't understand is that it looks as if the pushrod on the pedal has a hard link to the piston in the master cylinder, so how can it produce a soft pedal unless there is a compressible link in there?

I deeply studied how the modulator and ECU works and i know them by heart but i know too little about the servo as it's quite uncommon failure on a D2 so i was not interested in it (untill now ), i'm looking forward to see the outcome on your's. Fingers crossed to be that and the SH unit to work well

OK, Old skool here, but servo failure is usually down to a split in the diaphragm, often (especially on a petrol engined car) unless the split is massive, you don't really notice it as there is sufficient vacuum, with the engine running, behind or in front of the diaphragm (can't remember which) to assist the pedal pressure. But it gets picked up at MOT time if the tester tests it properly. This happened to me on Cortinas.
And Wifey's Porsche Speedster copy which is semi auto has one too which operates the clutch.
Remote servos were not, for a long time, the norm, the usual place for them was exactly attached to the master cylinder. So yes all connected to the same pushrod from the brake pedal. Morris Marina 1800TC, (though bizarrely not the 1800 single carb), (I have one in a kit car and wish it didn't have the servo, it is overbraked with it), Cortinas from about MK 3 onwards etc etc, very run of the mill cars for the time. They had little air filters in them made of sponge you were supposed to keep clean.
Kit cars builders who wanted one, but couldn't fit the standard one off the master cylinder, used to hunt around scrap yards and generally used BMW remote ones.

Tim C said:

Thanks for the link, as you say, maybe try the used one first! I haven't found one in the UK yet. I hope the issue doesn't get any more interesting and that another servo sorts it!
Rave p1054 shows a good diagram of a servo. The principle is fairly straightforward, but I can't remember the detail of how they work. The bit I really can't understand is that it looks as if the pushrod on the pedal has a hard link to the piston in the master cylinder, so how can it produce a soft pedal unless there is a compressible link in there?

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Here's a good explanation, it gets to the servo eventually!

Thanks for that. That is a great video, but it confirms what I was saying about a hard link from pedal to master cyl piston, so I am baffled as to how the servo can produce a soft pedal.
Logically it must either be air trapped somewhere which can only be compressed with the extra shove from the servo, or the containing pipework has to expand. The hoses don't bulge.
With the new master cylinder, the initial pedal is harder than it has ever been. There are no leaks. The sinking pedal suddenly started a month ago and did not gradually develop.
A new set of genuine bleed screws are on order. When they arrive I will bleed the system again just in case using it has dislodged any more air. I'll do it before fitting the servo.

Just another thought .....perhaps fitting a stop / start system would fix it! Then I would have a nice firm brake while sitting at the traffic lights!

Tim C said:

....it confirms what I was saying about a hard link from pedal to master cyl piston,...

Click to expand...

I'm clutching at straws with this but IMO the video is simplified to be easy to understand though if you watch it carefully you'll see that it's not a "hard" link . If you watch RAVE you can see that there is the input rod from the pedal then the ratio disc and reaction disc to the output rod so not direct link and it's stated:
"Brakes held on
When the brake pedal effort is constant, opposing pressures cause the reaction disc to extrude onto the ratio disc,
which moves the piston against the valve to close the air inlet port. This prevents any further increase in servo
pressure and maintains a constant output force to the master cylinder assembly."
IMO the gist of it is in the red parts for the increased braking distance as that output force can be not strong enough for some reason, .... about the soft pedal with engine running i rest my case cos i said it many times before, should not be a problem as long as the vehicle stops as it should but as your's doesnt the investigatiom must continue

yes the servo in a Discovery has 2 diaphrams, maybe ther an internal leak between them, do you have a copy of the RAVE manual

Tim C said:

Thanks for that. That is a great video, but it confirms what I was saying about a hard link from pedal to master cyl piston, so I am baffled as to how the servo can produce a soft pedal.
Logically it must either be air trapped somewhere which can only be compressed with the extra shove from the servo, or the containing pipework has to expand. The hoses don't bulge.
With the new master cylinder, the initial pedal is harder than it has ever been. There are no leaks. The sinking pedal suddenly started a month ago and did not gradually develop.
A new set of genuine bleed screws are on order. When they arrive I will bleed the system again just in case using it has dislodged any more air. I'll do it before fitting the servo.

Click to expand...

Surely if your bleed screws had a leak you would be losing fluid a lot of the time, yet you seem to have had no drop in fluid at the reservoir?
If you look at all the bleed screws while someone maintains hard pressure on the pedal you would see tell-tale drops of fluid coming out, or be able to hear/feel air coming out?
If you bleed a brake then leave the plastic pipe connected, i.e. full of fluid. Then close the bleed valve tight and THEN watch it while someone pushes hard on the pedal, if there is a leak you will see either fluid or air coming out. It can't just disappear and then reappear in the reservoir!
Bleeding brakes is a common problem on these trucks and there have been a few threads on it.
Not saying you haven't had a really good Google on it, but worth doing if you haven't.
In the old days you could test for a leaky/split diaphragm by taking it off then sucking on the vacuum pipe and seeing if it holds vacuum, by putting your tongue over the end of it to seal it. If it didn't hold then yes you deffo had a split diaphragm.
Dunno if this would work with a D2 if it has two diaphragms. Only one split may not show up on this test.

Best of luck!