Has the braking efficiency dropped off as a result, if so I would say you servo has failed, does it have a good vacuum supply?

The ABS is normally fail safe so shouldn't effect the brake efficiency even if its not working. Have you tried disconnecting the vacuum feed to the servo to see if it makes any difference to the braking? Does it feel as though there is no servo assistance?

Start at the vacuum pump, remove the hose, start and let the engine idle, put a finger on the outlet (or inlet as it is vacuum) from the vacuum pump you should feel the vacuum.

Refit the hose then remove the hose from the servo and you should feel the same vacuum as the vacuum pump.

I have had the hose separating internally and blocking the vacuum, or the non-return valve can be faulty.

The test for the servo - Switch off the engine, pump the brake pedal to exhaust all the vacuum in the servo, then hold your foot on the brake pedal then start the engine, the brake pedal should go down ½” if the servo is OK.

Let us know how these tests are.