A 38mm piston lays about 1133 sq mm. against the pad. For a 40mm piston it is about 1256 sq mm. That's roughly an 11% increase in braking pressure. Not a tremendous difference (especially on the rear wheels), but not totally insignificant either.

It makes sense that the wagons, with a higher expected weight loading on the rear wheels compared to the sedans, would benefit from a slight change in brake bias to the rear. I believe some of the sedans even used 36mm pistons.

Dear ajs,

May this find you well. Volvo VADIS - the now-superseded Volvo Dealer Parts and Service database - shows that 38mm and 40mm pistons were specified for 940s. I believe that the 38mm piston was generally used on sedans, while the 40mm was used on wagons.

Just as it is proper to replace tires in pairs - even if one is still serviceable - it is best to use matched calipers. As front brakes provide most of the capacity, 38mm rear wheel caliper pistons should be fine. I'd not want to have mis-matched calipers, except as a short-term fix.

Hope this helps.

Yours faithfully,

Spook

i believe either 40mm or 38mm were used on 740's and 940's which use these calipers.

from a safety aspect i really do not think the braking difference between a 38mm and 40 piston will cause a problem.