Let's expand on the conversation so everybody knows what we we're talking about & for the newer members to catch up. When I built my first buggy (Mary) I followed the tried & tested route of modified Mini front brakes on the rear of the buggy. This worked & passed SVA but anyone who has got these knows it's a poor handbrake. So when I built my second buggy (Mr Bumble) I decided I wanted better rear brakes so I used Golf rear callipers, modified the Mini hub slightly. This also passed SVA & to me was a far better solution than the Mini setup, it gave great braking & had a really good handbrake. I have since removed Mini front subframe (at the rear) & fitted the MGF subframe along with the MGF discs & callipers along with the K series engine which is the route that Danny has just gone down.
The problem I 'think' is the volumes of fluid needed in the braking circuit, the Mini M/C was designed to have either 4x cylinders or 2x cylinders & 2x callipers (using a Yellow tag). When the Golf rear callipers were fitted I don't think the amount of fluid need to make them work was much greater than was needed to make the Mini cylinders work, so they worked fine. But I'm convinced that the MGF callipers are a bigger diameter piston therefore to get them to move/work it needs more fluid. I believe the solution is to try to either find a M/C that has a bigger 2nd chamber for the rears, or see if it is possible to split the 2 circuits onto separate M/C's but I am not sure there is enough space.
Sorry for the ramblings, has anyone got any thoughts if I am on the right track?