I'm gonna fit Swift rear disc callipers because they have a handbrake fitted in them. I've looked at the swift MC and the dia is a little smaller, but the swift front callipers have a bigger cilinder. So I'm gonna try to fit the swift rear callipers and MC. If I'm thinking about this right, the swift MC should push enough fluid to the front callipers (as they're smaller) and also to the rear, because they are what originally is fitted to the MC. Am I overlooking something here? Or might this be something to try for you too James?

That should work OK as its a disc/disc unit, I'm a little surprised that the Swift m/c is smaller. Does it have a size marked on it? it may be just a different thickness body. The real problem with the disc drum master cylinder is that it doesn't push enough fluid to the rear calipers. That means not enough pressure and as a result the front brakes are poor too.
Theres a couple of reasons why I didn't try the Swift m/c. I think the Swifts were all Japanese built so they would have Jap brakes, the fittings are a bit different from my Lucas brakes. The local breakers isn't very big and the only Swift there was disc/drum. Rovers are a straight swap and are plentiful everywhere in the UK. They also depreciate really badly so the parts are very cheap 
I'm still looking for a slightly bigger than stock m/c (mine is 7/8) to go with the lwb calipers. Reading Mark and Tony's comments I am less bothered about it now but I would like to maintain the front m/c to caliper diameter ratio , a 1" should be ideal.
Have a look under the Swift and see if it has a combination valve, you might need that too. If you have too much rear bias then try some lwb brakes. Let us know how you get on.

Lol I forgot to mention only Swift GTI's had rear discs  but the Vit MC is 22.2 mm or 20.6 and the Swift GTI is allways 20.6. It should be logical that the Swift MC pushes more fluid to the rear, but I'll try and find out (will take me some time though.... maybe another christmas project  ). My Vit is Canadian build so I have to find out about that fitment too, but at least it aint spanish  We don't have as much rovers over here but if this don't work I'll dive into that direction too I think. Don't know about the combination valve but I'll ask around to find out 

Could the lack of pressure be the LSPV

I thought the pistons were the same size for
the front and rear brakes, ,unless there is some
sort of stepped piston in the M/C I don't see
how there could be with a single piston M/C

Could the lack of pressure be the LSPV

I thought the pistons were the same size for
the front and rear brakes, ,unless there is some
sort of stepped piston in the M/C I don't see
how there could be with a single piston M/C

The outside diameter is the same but the piston for the rear brakes has a rod in the centre of it to operate the front brakes in event of a rear brake failure. That means that its effective surface area is much smaller so it displaces less fluid. From what I've read the disc/disc m/c is a slightly different design. I intend to pull them apart to verify that as I haven't been able to find a drawing for a disc/disc m/c. I came across this yesterday while looking which is pretty clear on the subject.

DISC / DISC 

A four wheel disc brake master cylinder is designed to supply more fluid pressure and volume to the rear disc brakes than the disc / drum master does. This is acheived through an internal piston re design. The piston that feeds the rear brakes on a disc / drum master will run out of stroke, limiting the amount of fluid pressure and volume that may be supplied to the rear isc brakes.

The four wheel disc master re design delivers the extra needed volume and pressure to the rear allowing your rear disc brakes to function properly. If you attempt to use a disc/drum master on a four wheel disc system you will get poor rear brake function and experience a spongy brake pedal with a long pedal travel. 


That was taken from here:

http://www.mpbrakes.com/masters.htm


What that doesn't mention is that the front brakes are operated by the pressure in the rear brake circuit. So bad back brakes = bad front brakes too.

Question on the vented rotor swap. 

Do you only need the new rotors, pads and calipers from a 4-door?  Does the LWB caliper just bolt up to the same caliper bracket as the SWB one?  If so, and the master cylinder will supply enough fluid for this upgrade (not doing rear discs yet) I just might give it a shot. 

I always like to have better brakes.

Mike

Yes, but make sure you get the caliper mounts too,
those are different from non vented units

I just fitted the complete caliper assembly straight onto the SWB hub. I think thats what Wild means  you need both halves of the caliper.

Yes, that's what I mean

Bonfire night preperations and darker evenings have meant that haven't progressed very far with this but I managed to find time to strip both m/cs at the w/e. The Rover m/c measured 22.2mm (7/8) and the Vit m/c is  20.6 (13/16) so the sizes tally with the Jap m/cs. I wonder if Nicks has the larger m/c which would explain why his rear discs seem to work a bit better than mine.
As expected the construction of the Suzuki disc/drum m/c (bottom) is quite different from the Rover disc/disc m/c (top).  The Suzuki m/c has a small diameter shaft running through the centre and the green valve which looks like the residual pressure valve  is secured on the end of the shaft by a circlip. The spring rates seem a litttle different so it looks like the disc/disc m/c gives a longer stroke on the rear at the expense of less stroke on the front, that may be offset by the fact that the Rover cylinder is a bit longer (but the pedal travel would be the limiting factor). In my case the larger diameter of the Rover m/c will offset this, it would be interesting to see the internals of the Swift m/c. The Rover m/c just has two pistons and two springs, the piston for the front chamber has a slot in the middle and a pin runs through it to prevent the piston coming back into the rear chamber.
I have found out that Rover 820s and MGFs have a three port m/c so I am trying to track one of those down, should be a bit bigger too 



The other surprise was how little was holding the Zuk m/c together, you can see in the pic that the Rover m/c has two moulded in clips on the plastic cover and a circlip underneath holding the piston in. The cover on the Zuk is a press fit with no circlip underneath, so it relies on the mounting bolts to hold it together.

My Rover 820 m/c arrived today. Brand new £35 with cap  but its a 2 port, even though they told me it was a 3 port. It is a decent size though so I'm going to run it. I just need to get a splitter and re run the front brake pipes.