brake failue

your piece of machinery is likely to be a vacuum pump: on a petrol engined car there is sufficient vacuum in the inlet manifold for the brake servo, but there is no vacuum to speak of with a diesel - so you have a pump instead.

If I was you, I'd check the interconnecting pipe, and also see if there is a vacuum at the servo end of it when the engine is running, as you have a choice of a failure of the pipe, the pump, or the servo to give hard pedal symptoms.

Try to determine whether the oil is engine oil or brake fluid. Brake fluid tends to strip paint and engine oil picks up a lot of dirt. As it's bolted to the engine it is probably engine oil. A faulty seal on the master cylinder tends to leak brake fluid, from the servo, and also down the brake pedal. Since engine oil leaks are quite common you really need to check whether you are pulling a vacuum, or it's just a leaking air pipe letting in air, or a failed servo.

Dont screw around with brakes. You could die if you get it wrong. Take it to a proper repair shop