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How about some symptoms? What is it doing and why do you think it might be bad? The easy test is if your brakes are not working. Give us a little more info and we can probably help you.
You dont say which master, but if its brakes you will have spongy brakes and the pedal will go down to the floor. If you pump the brakes they will improve as to pressure but then the pedal will if maintained drop off when stopped.
If its a clutch, you will have hard shifting, no pedal pressure no matter what ya try.
Look inside the cab at the clutch hole and see if you find fluid on the floor in the carpeting, if there is, bad master. Look under the hood at the brake and if there is fluid dripping or evidence of leaking then you will probably have a bad brake master.
But you need to be more specific when asking a question like this.