1998 C70 230K, Went through a major maintenance cycle catch-up/repair/upgrade cycle last year. Control arms, end links, CV axles, brakes, brake lines, cooling system (ALL of it), timing belt, head gasket, engine bushings, ABS unit rebuild/replacement.

I've been getting some funkiness from my brakes and I'm not sure if this is a strange master cylinder problem, booster, or something else.

My main symptom is occasional "Stiff" brake pedal. Hard resistance at pedal, pedal very high. Car stops fine, but tend to "release" a bit once nearly or completely stopped. If it releases after I stop, the car will roll about a foot before the pads catch it... which is scary. The symptoms are mainly as described. I don't get "squishy" brakes or soft pedal. Brake are definitely working. But this stiff/release bit has got me concerned.

I initially feared it was a failing MC and I ordered a replacement ATE unit as soon as the strangeness appeared. I have not installed it because I'm now wondering if this is something vacuum related (booster, hose, or check valve), or perhaps something related the ABS unit.

I hate shotgunning money at problems, but with high miles, and an unknown number of years of neglected maintenance, many things have worn out.

Any thoughts?

1.) Does the car creep forward at the stop lights regardless of how hard you press the pedal?

2.) Does the pedal slowly sink to the floor as you wait at the stop light?

If you answered 1.yes and 2.no, then it almost sounds like your ABS is screwing up. Any warning lights?

neither actually.

If it is the check valve in the vacuum line going to the MC, the brake pedal will not go down as you start the car. I suspect vacuum problems, not the MC.

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My name is Klaus and I am a V♂lv♂holic

yeah, that's what I have come to suspect. any good way to differentiate between the booster itself and the hose or check valve?

Not really. If the check valve is toast, there will not be enough vacuum. If the booster seals are bad, all of the vacuum in the world will not help. It is just that the check valve is easier to replace than the booster, cheaper too.

IIRC the check valve is under the air cleaner? Been a while.
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My name is Klaus and I am a V♂lv♂holic