If there is reduced Vacuum you Brake Booster has reduced power to do the job and that willl make the Braking effort harder.

I doubt if Engine Oil made it all the way to your Brake Booster. But, you could unscrew the vacuum tubing connection at the Brake Booster and see if there is Oil. Or, you could loosen the Master Cylinder retaining Nuts and scoot the Master Cylinder forward a little and see if there is Oil in the Brake booster.

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84 300D, 82 Volvo 244Gl Diesel