I’m thinking of rebuilding my Roadrat back to singlespeed/flat bar mode a simple ‘knock about’ bike, probably with a 29″ MTB wheel setup, and thorugh that it might be good to put the old style pedal back brake on it with no front brake. I had pedal back brakes on a bike as a kid and can’t really think of a downside – as powerful as you need them to be, maintenance free as far as I can remember, I don’t seem to remember them being affected by the wet or mud. Can you still get these?

you can, although the hubs tend to be a bit weighty.

Sheldon on coaster Brakes: http://sheldonbrown.com/coaster-brakes.html

I Think you need a frame that’s adapted for them, don’t you as there’s a reaction rod on the hub?

Look at the SRAM duomatic, a 2 speed automatic hub with a coaster brake.

They are still available. As WWW says, ‘coaster brake’ is the term to use when searching. I’ve got an Sram Automatix (two speed, automatic, coaster brake) hub on order at the moment for my Moulton, and it has a Sturmey S2C fitted at the moment.

Your bike will be illegal if the coaster is your only brake, though. You must have two independent braking systems in the UK. They can be on either wheel, although common sense would dictate that you should fit a front brake, rather than a second rear.

ETA: downsides:
you can’t pedal backwards, which can make getting the pedals to a good starting position awkward – a bit like fixed, really.
Until you’re used to it, you will get the occasional unplanned stop when you go to back-pedal on the move.
Power varies depending on pedal position; I tend to get the cranks vertical for drag-braking, which then limits the power available if you suddenly need to brake harder. Conversely, with the pedals level it can be difficult to brake gently… It’s all practice, though.
Removing the wheel is a little slower, because you have the reaction arm to deal with.

Thanks. Good point on the reaction arm, I don’t seem to remember that from my childhood, but then again I remember that all summers were sunny and warm and all winters were snowy, so I might be looking back with rose tinted spectacles!

Not too bothered about the not pedalling back thing as its mainly a knock-around bike so I wont be doing any serious riding or commuting on it, i’m sure i’d get used to it. If not i’ll just use the Canti brakes.