I recently upgraded the brakes on my 1965 ford mustang. I replaced the original front frum brakes with disc brakes. I wanted to know if I would also need to change my master cylinder? Thanks.

yes- change everything- including brake lines- get it done RIGHT- you might be needin' good brakes-

Combination valve/ rear metering valve as it applies would give the metered amount of pressure to the rears and divide the brake pipes giving them a place to attach. if you purchased an update, it should take this into consideration and provide the correct safe master and valve. You might have purchased the rotor/caliper portion, so far, but need the other parts.

Having different parts that may be more effective, can affect the stopping and when correct, prevent unwanted lock-up, or sending the occupants toward the windshield on boost assisted braking can be averted.