now then..... the rst master cylinder will only be able to pump X amount of brake fluid, IF the size of the new caliper pistons are larger, they will obviously need more fluid to fill them when you press the pedal - if the master cylinder in a st170 is larger than a rst - this will explain it. you might need to have a check - that is the diameter of the pistons in the st170 calipers compared to the rst ones?