leaking master cylinder fluid reservoir due to cracks in reservoir

Yes you should be able to replace the reservouir quite easily, and yes you will get air in the master cylinder or at least you should assume you will, and bleed the system appropriately. You don't mention whether it's the clutch or brake system though, not that it really changes things.

I've not tried braking with the left hand pedal for a while but ISTR its a lot better with the middle pedal!

Grr, I meant bleeding the bleedin' system!

Although I suppose that attempt at clarification is just going to make matters worse..

In most cases, probably all if you think about it, the reservoir is above the cylinder and any air bubbles at the cylinder inlet should disperse naturally into the new reservoir. If you don't work the cylinder the risk of getting air into the system is negligible.

But bleed it anyway, it takes minutes and it's daft to miss it out. Given that the OP is asking these basic questions, I suspect he's not familiar with the car and may well end up experimenting his way into needing the brake system bleeding.

And as a further 'enhancement', at least partially depress the pedal before starting any work and keep it that way (piece of wood between pedal and the seat or seat base) until you are finished. That way the pressurised part of the system is totally isolated from the reservoir (unless the piston seal is knackered) so can't get aired and returning fluid when the pedal is evantually released will flush out the air that didn't get in in the first place.

Thanks for the tips, especially about depressing the brakes while working. It is the brake master cylinder, the car has an AT.

I can't seem to find JUST the plastic reservoir for sale, but rather entire master cylinder assemblies.

Also, you have said it replaces "Easily." How? Does it just pull straight off the master cylinder? The manual shows that you work it off by rotating it to the right...this seems iffy to me and likely to cause it to shatter in my hands.

Is that a problem? You are replacing it and it's in no fit sate to reuse. Siphon out the brake fluid so it doesn't make a mess, from what you say about the reserviors condition that might be a good ruse anyway. Sharp plastic splinters, not good, wear gloves.

No it's not a problem if it breaks. What I am mostly worried about is it leaving the tube/stubs stuck into the master cylinder where I can't get them out.

I'm a RRC man not Disco 1 - is this a vacuum assisted system or the pump pressurised one?

Vacuum assisted with the big black vacuum "servo" behind the master cylinder.

I think that you're in luck on two counts then:

The reservoir is available for this setup - it's s/n dependant: any with ABS - STC2904 to LA081991 w/o ABS - RTC5833 from MA081992 w/o ABS - STC2902 but obviously confirm these yourself. I'm not sure that you can get the reservoir separately for the pressurised system (which may not have been used on the Disco, anyway).

Getting the old reservoir off 'should' be a doddle. Loads of room to manoeuvre the old one off. Reading between the lines for the other system there may not be enough headroom. It's a straight pull off/push on but a sideways rolling motion/waggle as you mention will help. You shouldn't have trouble with 'tube/stubs' getting stuck. There's a big juicy rubber seal between the bits. Take care not to drop dirt down into the master cylinder. Clean off externally first and when removing the seals take care to keep bits thereof out of the master cylinder. The two seals are different. Remember that brake fluid and paintwork don't mix! Wash off quickly with water if you spill any fluid.

:) Thanks!