Can we assume there's a FAM7821 brake valve hiding under the servo? If there is your only options are smaller wheel cylinders or replace it.
Of course, lacking ABS, there's always the possibility of locking up the rear particularly on loose or uneven surfaces. Start by servicing and checking the rear brakes, handbrake cables... They can get out of balance pretty easily.
"IF there is, your only options are smaller wheel cylinders or replace it". it - as in the brake valve? I had the rears serviced by myself and a mini maniac guy I know. Pads were evenly worn, one of the retention springs were put in wrong causing them to wear so we replaced that. Gave the whole area a proper de-dusting and clean. Checked for leaks etc. Handbrakes cables also looked at. Holds tight at 2-3 clicks. Did that all according to haynes.
Ill try some moderate full brake scenarios this weekend/over easter to find out if it was a one off thing or not, which direction it spins, if the tires lock etc. Still need to read up on calverst' article.
