'pretty much the only thing to effect your symptom-ology.
AND the MC / Booster conniption is certainly possible.
The pedal travel issue points to the MC.
BUT the ABS ain't happy with SOMETHING.(Or the ABS is ill)
[do we have any diagnostics that can be applied to the ABS?]

I swear, I'd still wanna verify the brake hoses inner lining has not de-laminated
and/or started acting like a one way valve internally.
(How old are the hoses ,and from whom did you acquire?)

The Hit man is/was correct.If pads/rotors are not properly "seated"against each
other problems result.Except for Ceramic pads,the bedding in process is an
increasing speed use of the brakes (without ever coming to a complete stop).
[But,if you're reusing rotors they must be "deglazed"as in X man's instructions]
(Bedding in is the transfer of materiel from the New pads to the rotors surface,
AND cannot properly be accomplished without a fresh rotor surface.)

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'84 300SD sold
124.128