As the servo amplifies the braking effect over the WHOLE system it makes no odds what size servo you have fitted with regards to the balance of the braking system.
The key here is the effectiveness of the rear brakes. The same rear drums and shoes have been fitted across the range, whether specified with a compensator or not. The difference that will have an effect on the drums is whether the slave cylinder is correct. A larger item is fitted to compensator-equipped cars. It is likely that Jules has inadvertantly replaced the non-compensator item with the larger rear wheel cylinder, designed for the compensator equipped system. This is probably why the rears are locking. As it is larger it would also be making the system feel like the brakes haven't been bled - there will be more pedal travel due to the extra fluid displaced to the rears.
Jules, what brakes are you running on the front (239mm, 256mm or 280mm?) and what do you have on the back (discs or drums?)? Do you have the part number on the GSF invoice, for the rear wheel cylinders? Is the master cylinder still a 1.3 item, too?
Bear in mind that the auto-compensator isn't the only way of doing it - you can fit a manual bias valve as an alternative. If you don't have an auto-compensator fitted at the moment then you're going to have to make up a bundle of brake pipes and if your fronts are crusty it's an excuse to remake them to ensure good braking and no MOT fails on brakes for a few years.
WYATT - lines from the front go in the bottom of the compensator, lines to the back brakes go in the top ports.
