Disco 2 Servo problem

Install a vacuum gauge, to be sure, but is it possible you have a problem with you ABS Block, watch this video that show line pressure is feed back to the master cylinder internally in the block, this may be indicated by your pedal going to the floor as the amount of fluid in the brake line is reduced by the ABS module, and as you already pushing on the brake pedal no more fluid is added to the system so the pedal keep going down, to make up the fuid loss, if this is the problem no amount of bleeding will cure an internal bypass of brake fluid if the solenoid is at fault or the valve it controls that is bypassing fliud, you will need to replace the solenoid/valve or the complete unit,
also, you need to check your wireing for you solenoids are OK, and that they are operating correctly,
I have put 4 links to the ABS all worth a watch

Thanks for all of the above. The bleed screws are just for a cheap elimination and nice to have a tight grip in the spanner! Thanks Frostythor, what you say about the modulator makes sense.
I'll try cleaning the plugs with contact cleaner first, but suspect a replacement unit is the answer. With a new unit at about 2 grand, a used unit at £60 might well be worth a punt!

Just to reinforce some of the earlier comments a brake pedal sinking to the floor CAN NOT be caused by the servo !

I used to be involved in the design & development of servos & master cylinders at Girling, so here's a bit more background detail for anyone interested :
The master cylinder piston travel from brakes off to full stroke / pedal to the floor is 36mm max ( I can't remember if it might be 32mm on a D2).
The servo typically has a total maximum internal stroke of approx 40mm - to ensure that the mastercylinder can always travel full stroke while bleeding (this is the same for single diaphragm or double diaphragm servos).
It's physically impossible for the servo to "lose" more than about 3mm of internal stroke (maybe 12mm at the pedal) unless internal components are actually missing (causing much larger & very obvious problems !).
The rubber "reaction disc" inside the servo (mentioned in earlier comments) can only extrude about 2mm max until its in contact with the "ratio disc". With the engine running & vacuum applied this happens at very low pedal force. With the engine off & no vacuum, the extrusion happens at much higher pedal force.
The exact clearance / extrusion gap is designed to give the initial braking characteristic of the car. Bigger gap = more braking with very little pedal force, very small gap = minimal braking until a lot more force is applied.

As other have already said, the creeping pedal problem is either the replacement master cylinder leaking internally or as also already suggested, maybe the ABS modulator leaking internally.

Please dont rely on rudimentary cartoons about the principle of ABS operation cos it has nothing to do with how the WABCO D modulator and ECU is working, dont bother to check wirings cos if it was something electrical you should have got warning lights and fault codes for that. I"ve spent the last 10 years studying how the D2's electronic systems are working and i'm trying to share what i know with those who are in need so you can believe me or not, first of all the D2's system doesnt send any fluid back to the master cylinder, it has two valves /wheel and the pressure is managed within the modulator, if you want to know how the D2 works better rely on the workshop manual not on youtube videos or google, here's a "flavour" about the D2's ABS activity (from the WSM):

"ABS braking mode
When in the normal braking mode, if the SLABS ECU determines that ABS braking is necessary, it energises the inlet and outlet solenoid valves of the related brake and starts the return pump. The inlet solenoid valve closes to isolate the brake from pressurised fluid; the outlet solenoid valve opens to release pressure from the brake into the expansion chamber and the return pump circuit. The brake releases and the wheel begins to accelerate. The SLABS ECU then operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the brake and apply the maximum braking effort (for the available traction) without locking the wheel."

Thanks Pawl, that's really useful to know. The original cylinder and the TRW one both show the same problem, so that leaves the ABS unit. The even way the pedal sinks feels just like fluid seeping past a seal or valve. I shall be happy to keep the used servo as a spare, when it arrives.
My ABS unit has number SRB 500050 03 025741 I've found a unit with number SRB 101241 03 008338. Using my considerable powers of deduction, I conclude that the 03 part might suggest that it also comes from a 2003 car? At £60, it seems well worth a punt! I'm hoping a change of ABS unit won't offend the ECU? Should I fit the shuttle valve repair module off mine, before I fit it?

tim if you going to replace the unit any was watch this video, if you try it out and nothing improves you can still replace the whole item , the one thing that got me was on the Altlantic video he said the problem was caused by water in fluid that corrodes the pins and so reduces the voltage

Another good video!
I've ordered the used ABS lump as it's from a 2003 ES manual, the same as mine. At £60, if it isn't the answer, I'll be happy to have a spare!
The ABS electrics are well beyond me, but Island 4X4 list the valve block @ £400 new. In their picture, the hydraulic side looks pretty simple with just two valves. Is it not possible to just reseat the valves?

Tim C said:

My ABS unit has number SRB 500050 03 025741 I've found a unit with number SRB 101241 03 008338. Using my considerable powers of deduction, I conclude that the 03 part might suggest that it also comes from a 2003 car? At £60, it seems well worth a punt!

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That 03 after the part number is not about the MY of the vehicle, the part numbers which had 101... after SRB were all fitted from factory to vehicles up to VIN ... 3A999999 but it's the same part with several superseded part numbers like in the case of the master cylinder. My advice is to wait with the modulator untill you see the braking with the replacement booster cos the main problem of your's is the braking distance so if that will improve and the EBD will not kick in like it does the sinking pedal symptom will not dissapear even if you fit a bran new modulator and that's due to the expansion chambers...

Tim C said:

I'm hoping a change of ABS unit won't offend the ECU? Should I fit the shuttle valve repair module off mine, before I fit it?

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If you decide to replace the modulator too don't worry cos it doesnt offend the ECU but it would be good to fit the modified SV switch pack to it and do the wiring mod from the beginning to not get SVS faults in the future(unless the switch pack fails)... IMO if it was something within the modulator(for this symptom should be at least two outlet valves for the same axle stuck open) then the EBD or ABS would not work normally as well ... Necessity of the mod = SVS fix(wiring mod option B) "graphic" explanation

Tim C said:

In their picture, the hydraulic side looks pretty simple with just two valves. Is it not possible to just reseat the valves?

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Those are the shuttle valves which have absolutely no effect in the pedal's travel or braking action whatsoever and a failure in that area brings on the 3 amigos for sure, the ONLY purpose of the SVS module is to tell the ECU if the pedal is depressed or not and if they leak that's visible cos fluid will be lost through the switch pack... i know that i may sound arrogant from time to time but that's because i spent hundreds of hours studying these systems(in theory, on the bench and live with tester connected) and it really gets me when i see that i'm trying to help in vain. Maybe others with youtube expertise know better. Good luck

Sorry Sierrafery, until this morning I thought I was replacing the servo, so haven't been reading modulator threads!

In your other thread, someone suggested that you check the vacuum. I don't know if you've done this already but it seems to me that air is not being removed from the servo and it's entering the brake circuit. With a multipurpose kit like this, you'll be able to verify the functionality of the vacuum pump, the check valve and the brake booster as well as for leaks in the vacuum line - [Sorry, for some reason URL pasting is not working. Just go to Amazon and search with ASIN B01N57K1JU]

The system can be tested as shown here - https://axleaddict.com/auto-repair/Bad-Brake-Booster-Check

NPG said:

I don't know if you've done this already but it seems to me that air is not being removed from the servo and it's entering the brake circuit.

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I really don't see how this could ever happen. The servo is a purely mechanical/vacuum aid to increase foot pressure on the pedal. There is no servo/brake fluid interface.
As far as I can see.
But then I have never stripped down a D2 servo.

Stanleysteamer said:

I really don't see how this could ever happen. The servo is a purely mechanical/vacuum aid to increase foot pressure on the pedal. There is no servo/brake fluid interface.
As far as I can see.
But then I have never stripped down a D2 servo.

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I too was sceptical about that but was mentioned numerous times by professional mechanics on the web (such as para 2 in both these articles) -

https://www.repairsmith.com/blog/brake-booster-check-valve/
https://oards.com/brake-booster-check-valve-function-and-bad-symptoms/

My point here was that having eliminated nearly everything, I would check the vacuum system before touching the servo and the modulator. It's in fact one of the first (and easier) things to do in brakes diagnostics.

NPG said:

I too was sceptical about that but was mentioned numerous times by professional mechanics on the web (such as para 2 in both these articles) -

https://www.repairsmith.com/blog/brake-booster-check-valve/
https://oards.com/brake-booster-check-valve-function-and-bad-symptoms/

My point here was that having eliminated nearly everything, I would check the vacuum system before touching the servo and the modulator. It's in fact one of the first (and easier) things to do in brakes diagnostics.

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Sounds like the Sherlock Holmes thing "Once you have removed all the possibilities then you are only left with the impossible"!
And I do agree with you that checking for vacuum has got to be one of the easiest things to do.
I'll have a read of these two!

NPG said:

I too was sceptical about that but was mentioned numerous times by professional mechanics on the web (such as para 2 in both these articles) -

https://www.repairsmith.com/blog/brake-booster-check-valve/
https://oards.com/brake-booster-check-valve-function-and-bad-symptoms/

My point here was that having eliminated nearly everything, I would check the vacuum system before touching the servo and the modulator. It's in fact one of the first (and easier) things to do in brakes diagnostics.

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So having read this it does sound like it could be the check valve.

OK, i've read the statements in those links but there's no explanatin how would that happen and watching the booster's scheme in the wsm IMO a check valve failure can't introduce air into the master cylinder whatsoever. From pure technical point of view the lack of vacuum in the output housing means that it's ambient pressure at the back of the cylinder and if air can get in at that pressure then fluid should come out before that, even more if vacuum is present so the booster would be full of brake fluid and the tank empty... if somebody can contradict that with solid arguments i'll stand corrected, a simple statement withut logic of a guy on the web is not enough for me, sorry.

Again, thank you all very much for your help. I am lucky enough to have a garage based in a farm unit 400 yard away from the house. The owner is now back from holiday and has just had a look. He said that the servo is definitely working, but thinks it is probably a pin prick in one of the two servo diaphragms.
He knows the car well and has MOTd it for the last five years and agrees it's a new problem. He said he wouldn't like to live with it like this himself, but curiously it would pass an MOT.
Just to summarise, the pedal is rock hard until the engine starts. Keep your foot on the pedal, start the engine and the pedal starts to slowly drop. Where I was wrong is that although the pedal drops a worryingly long way, it does in fact go hard at the bottom and will stall the car. (I think Sierrafery said that his does this too)
He said he has only met this twice before, once with another Disco 2 and the other was a high mileage Golf Mk 5. In both cases it took some head scratching!
Unfortunately I still haven't tracked down a new servo, so will fit the used one which came this AM. Hopefully the used modulator will go on the spares shelf when it arrives!
Obviously the only way to be sure is to try it and hope the replacement doesn't share the same fault. I will report back!

Tim C said:

Keep your foot on the pedal, start the engine and the pedal starts to slowly drop. Where I was wrong is that although the pedal drops a worryingly long way, it does in fact go hard at the bottom

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That's exactly how they all behave and it's normal IMO cos the expansion chambers have air in them(so they are built), at this point all i can think is that the booster doesnt deliver enough pressure to the cylinder for some reason so the ECU activates the EBD cos the deceleration rate is not as expected at that speed. As i said there is a quite unlikely but possible scenario that the outlet valves in the modulator are stuck open or not closing well but then i think the EBD would misbehave as well... we'll see cos sooner or later your saga will get to an end

sierrafery said:

That's exactly how they all behave and it's normal IMO cos the expansion chambers have air in them(so they are built), at this point all i can think is that the booster doesnt deliver enough pressure to the cylinder for some reason so the ECU activates the EBD cos the deceleration rate is not as expected at that speed. As i said there is a quite unlikely but possible scenario that the outlet valves in the modulator are stuck open or not closing well but then i think the EBD would misbehave as well... we'll see cos sooner or later your saga will get to an end

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So Sierrafery, if as you think the oultet valve in the modulator may be the problem what would you do to retifie this

frostythor said:

So Sieafery, if as you think the oultet valve in the modulator may be the problem what would you do to retifie this

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I dont think that's the problem, i said that "is a quite unlikely but possible scenario" in theory but i didnt see it in reality untill now and i've seen a huge variety of D2brake system faults . However, if that's the case the modulator must be replaced with known good one