Many years ago I discovered coolant and oil mixing in a GoldWing so I reasoned that it must be a water pump failure (I didn't understand the function of the weep hole at the time). I replaced the pump and the front cover gasket and made an extra trip to get allen head bolts to replace the original philips head ones, put everything back together, filled the oil & coolant, started it up and watched it pee green out of the exhaust. That's when I realized that the oil & coolant could mix if a head gasket was blown too. Another week and a half of delay while the gasket came and I pulled it apart again.
This was my winter sidecar machine and I had to get a ride to work for almost a month while it was off the road and I had to heat the uninsulated steel garage enough to work on it. When the gasket blew again a couple of months later I gave up on that motor and bought a good used replacement. It cost less than I had spent for the parts I put into the original engine and needed almost no work until it's water pump went about 10 years later.
In those days we didn't have internet forums like this with people who have already done what you need to do and can tell you what to do to avoid a mess.
BTW: You can probably find a good used master cylinder at a local wrecker fairly cheap. It doesn't have to be from the same model as yours or even from a Honda as long as it is for a model with the same number of calipers as yours, but I would look for one that you can use your brand new brake light switch with.