Brakes question
 

It rained today and on one stretch I pressed the brakes harder than usual and front wheels locked right away.

I can't tell if the rear brake at all.
Is this normal for these cars and is there a way to adjust this?
4 almost new Michelin tires and new pads\ rotors

I find it pretty normal, rain, snow ice, sleet, whatever, the fronts always lock. 108/114/110 they all dive a lot. and most of the weight is on the front. once in the middle of the night tried to brake my W110 on ice, slid right through the intersection, was pumping the pedal as far as like 200 yards from the red light, luckily there was no one else there.

The brake bias is fixed. In the wet the fronts will lock first. In the dry it's usually the rears. An adjustable proportioning valve would have been nice, but I doubt most of the original buyers would have been knowledgeable about or interested in using one.

Practice your threshold braking.

or you have a sticky caliper , just keep an eye on it, feel the rims , make sure one isnt hotter than the other after driving

Brake flex hoses in the rear could also be collapsed internally, leading to little/no braking on either or both rear wheels.

If pads and rotors have been changed, and the flex hoses are older than one can verify, then they ought to be changed out too. Places like NAPA can get them in.