Many of the 164s (including mine) don't have vented rotors, in fact they
are probably just like yours.
Note that the Volvo power brakes are not the "hair trigger" type as used
on GM cars when power brakes first came out.

Be sure your booster is OK, getting full vacuum, etc and that it is not partially full of brake fluid (from a leaking rear master cylinder seal).
That will put a cramp in your braking. Cracks in the vacuum hose or an
ineffective check valve between the manifold and the booster will also
hinder brake operation.

Vented rotors don't really improve initial braking. They just slow down
brake fade from overheating.

Are all your brakes retracting like they should? If not, you may have swollen
brake lines. This has been a chronic problem with the OEM brake lines
because of their small inside diameter and the propensity of the rubber to
swell. It turns them into a check valve so the brake applies OK but does
not release and continues to drag. I've ruined loads of pads and a couple
brake disks that way before finally learning about the problem (thanks to
the guys at Voluparts in Atlanta).
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma