With the Yellow band types, the rears are normally connected to the lower port.
I have a yellow band, drums, and the lower cylinder is connected to the fronts. It was always like that since that MC was fitted by a garage of dubious competence in the early 90s. The lower half of the master cylinder does look like it has a wider bore (as Ethel mentioned). If I connect them the other way around, will I get a higher mechanical advantage on the front brakes? But this is probably not something I want as pedal travel is the more limited resource.
However I note Spider says "normally"; are there cases when this is not true?????
As a straight forward set up, yes, the lower port on the Yellow Tag Master would connect to the Rear Brakes, as Ethel and Mad4 has mentioned.
The part of the Cylinder that the Lower Port uses has a larger diameter (from memory 19.00 mm), so this will displace more fluid but be at a lower pressure that the upper port (the cylinder here from memory is 17.00 mm). By having a Master Cylinder configured (and connected) in this way, it reduces the pedal travel while getting enough pressure for the front brakes to operate satisfactorily.
OK I have tried this, and the results are as predicted, but quite dramatic. Connected up the wrong way around, as they were on my car since 1992, the pedal tends to move alarmingly far without doing anything (as all the travel from the narrower bore part of the cylinder just goes to the rears). Then it feels like it's practically hit the floor, but you are now operating the front brakes, which are lower travel and higher effort than they should be. I always pumped the pedal a bit, both to get some confidence that it wasn't going to hit the floor, and to get my foot where I wanted it for "heel and toe" (I like the brake above the throttle). Once you have got it pumped up you have a very firm pedal and you have to push it quite hard. Ultimately of course the brakes work fine and as well as they were ever going to.
With them connected up the correct way round, the pedal has a much more natural and progressive feel. You just press it and the car slows down, and will lock the wheels if you want, just like any other car. It's quite light by comparison but with more travel. It also seems to be harder to get a higher pedal with pumping. Not sure why this is, perhaps because pumping (which has to be done quite quickly) tends to work more on the first part of the MC because it's nearer. I appreciate that this pumping business is a bit weird and a quirk of my driving style (in this car). But it worked for me.
Anyway it is now harder to heel and toe, so I may actually change them back, but will see how I get on, and am experimenting with other solutions. Just putting a bit of plywood under the carpet seems to work quite well-- my requirement is just to keep my actual heel on the floor at all times. I know some people can heel-and-toe with actually lifting their heel off the floor, which helps with a lower brake pedal, but I think you get much better control if you keep it on the floor. Or maybe I'm just lame.
Thank you to Spider, Minimad, Ethel and everyone else. This has solved the mystery of why the brakes always seemed so strange!
They sound as if they were positively dangerous before. I'd not be going back to that setup however hard it makes heel and toe the correct way round.
