Is replacing the brake master cylinder very similar to replacing the clutch master cylinder on most vehicles?
There are more parts to the brake system. Such as four slaves instead of one- so it takes more bleeding. The brake system also (almost all the time now) has a booster that is operated by engine vacuum. This is often replaced when the master is done. The master cylinder on the brakes often has two or more lines attached to it. The master cylinder on the clutch often only has one. The actual work needed is four of more times then that of replacing the clutch cylinder.
Similar but there are more lines to connect and bleeding is more involved.
The clutch master cylinder has only one slave cylinder to operate, so only one outlet pipe to connect and only one circuit to bleed. The brake master cylinder is much more difficult: 4 or 6 pipes to connect and circuits to bleed; they all must work or your brakes will fail with severe risk of death! The brake master cylinder will be attached to a vacuum booster which might be bad as well; the activation rod from the brake pedal, through the booster to the master cylinder might need to be adjusted. Finally, the brake master cylinder may have several electrical connections (brake fluid level, brake lights, brake balance warning light, ABS antilock braking system) that must all be correct. Conclusion: car owners who are not also trained mechanics should take their vehicle to a professional for brake master cylinder testing and replacement!