Series 3 Brake fail - update

Folks - would I be right in assuming that if the brake pedal now travels to the floor with almost no braking action apparent but there is no fluid loss anywhere, reservoir full, no bulging pipes that means my master cylinder has gone??
Servo assisted dual line system.
No brake work been done for a year so nothing new or changes recently. Used regularly so not been sitting out. Fluid changed approx 18mths a go.
Despite a new m/c being fitted only a few years back I’m thinking it has to be the cause - but anyone think of something else I could try before fitting another new master cylinder?
Thanks.
TS

If you pump the pedal does the servo hold it up ?

I think the test is to pump the pedal, hold it down, then start engine, pedal should then drop further ...

There’s really no pumping to be done. When I press it it now moves prettt much straight to the floor tho if I hold it there and start the engine it does sink an extra bit but only about a centimetre...
It wasn’t as bad yesterday but on repeated pressing and adjusting shoes to see if I could improve things it just got worse and now no resistance at all...

Assuming wheel cylinders not leaking and master reservoir is full then agree it’s the master cylinder, if you remove it and dismantle you will prob see a faulty seals, ensure you get a decent one TRW or Girling more expensive but last longer

If you see the fluid has become discoloured / cloudy it's a sure sign of likely seal fail in master. When fixed pump fresh fluid through the system.

Are you on drums? Check all round and make sure one of the shoes hasn't given up.
Re. pumping the pedal, do it several times quickly to see if it pumps up, with the engine off ie no servo.
Do you have a friend who can pump the pedal for you? If so then take the cap off the reservoir and check if pumping the pedal is stirring the fluid round

TS2304 said:

There’s really no pumping to be done. When I press it it now moves prettt much straight to the floor tho if I hold it there and start the engine it does sink an extra bit but only about a centimetre...
It wasn’t as bad yesterday but on repeated pressing and adjusting shoes to see if I could improve things it just got worse and now no resistance at all...

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Is the fluid still clear or any black bits in it?

Have checked the shoes and they are all good. Can’t get any pressure no matter how much pumping engine off or on.
Can’t see any black stuff in reservoir and liquid looks a little murky but nothing I wouldn’t expect from oldish fluid (Was due to change it in next couple of months).
Pressing peddle with cap off produces some fluid movement but not sure I’d describe it as ‘stirring’. Should it be completely static??
One of my rear cylinder bleed nipples seems to be jammed right now so have to unstick that before I can try to bleed but as haven’t worked on the brakes in ages I can’t see how air could have got in anyway so amn’t hopefully that would help anyway but worth a try I guess...

Bung a new mcyl on it, I use the cheapy britpart ones on my 88, they last a few years then tend to seize up due to lack of use, but at just over 10 quid who cares?

Think I’ll have to but the dual line ones are closer to £50 sadly and until I can unstick my bleed nipple I can’t replace the m/c anyway as I won’t be able to bleed it clear afterward. Unless anyone knows a way to replace the m/c without getting air in??
If I end up needIng to put in a new rear cylinder too then all of a sudden I have a hefty bill but seems that might be only option

Update:
I saw in the workshop manual that it suggested testing the m/c by clamping the brake hoses. So i clamped front and rear and tried the peddle expecting it to hit the floor as before. Lo and behold it was firm as a rock and hardly moved at all. Pointing, despite my initial thoughts, to an OK m/c.
Released the front clamps and still decent peddle.
Released the rear clamp and peddle hit floor. Clearly then an issue at the rear...
Managed to unseize the bleed nipples at the rear and attach bleed hose. Air comes out. A fair bit. Both sides.
Tighten up and the brakes are back working - kind of. One of my front cylinders won't return fully (another seized bleed nipple there too which I can't seem to shift yet).
SO... it seems my m/c might just be OK, though when the fluid in the reservoir did settle overnight I could see today that there is indeed black powdery stuff in it so I'm thinking the m/c is probably on its way out too.
At least I have a landy that stops again til I can go through and probably replace all cylinders and fluid in due course.
What I don't understand is:
How can air have got in despite no work being done on brakes for months?
How did peddle not go 'spongey' but simply stop having any resistance?
And how the feck do you stop the bleed nipples from seizing up between fluid changes??

TS2304 said:

And how the feck do you stop the bleed nipples from seizing up between fluid changes??

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I find putting a smear of red rubber grease on the threads helps. It also keeps air out during the bleeding process.

Are they cast iron or aluminium wheel cylinders? If they are aluminium, they steel nipple can often seize due to dissimilar metal corrosion.

Check master cylinder & make sure seals haven’t perished. Happened to me with clutch master despite having a new cylinder. Seals were perished. Also you say you changed brake fluid. Was it new type rather than mineral? It could have damaged the seals

If you’re stuck you could try slackening off the feed pipe it goes to the same hole in cylinder
Constant light tapping with 1/2 lb hammer on nipple May loosen it with release fluid, I use copper ease on threads
Just recently changed rears with brass fittings which should help also

To stop bleed nipples seizing up you need to fit the little rubber or plastic caps, but the important bit is to put a smear of grease in the cap before you fitt it to the nipple.

Cheers for the suggestions folks.

Regarding changing out your MC if you cannot bleed a brake cylinder and don't want to replace just now.
You can disconnect the brake pipes and immediately cap them with tight fitting plastic caps so as to not let in significant amount of air. You then bench bleed your MC, bolt to firewall and reattach the brake lines. Should give you fairly decent brakes.