That is why also that your being careful not to lock up the brakes. Your MC is designed for front discs and rear drums. MKnittle suggested a GM or corvette style MC. Now, speaking from experience and all the research I had done, although the GM style MC has equal split reservoirs, don’t buy the first one you see. Make darn sure it is made for front and rear discs. When I converted to hydroboost the MC I got in my kit from Tallon Hydraulics/Vintage Speed was a GM style with 50/50 split reservoirs. But in the description it says for front disc and rear drums. The difference(and I had to drain both reservoirs to actually see the difference, because I needed to know why another identical MC was labeled for front and rear discs) is in the tiny holes at the bottom of each reservoir. The one that controlled the fronts were larger in size(although still a tiny hole) than the rears for the drums.
The reason our factory MC have a larger reservoir for the front discs is as the pads wear(much faster than rear drum shoes do) more fluid is needed to compensate for the gap needed to the rotors as the shoes wear, so fluid level will drop. Most of your braking is done with the fronts on a disc/drum system, hen the drums catch up to balance the braking.

Right now your rears are braking way sooner than they are suppose to, thus causing the lock up.

If you ever want to convert to hydroboost, do a search on hydroboost and read up on the 4 write ups from MKnittle, ThrashingCows, JFonder, and Oochieman. They have listed part numbers, vehicles from the local Upulls it to grab the boosters from, etc.

If you want to learn more about MC types, look up hot rod forums as I did, and read up on GM and corvette MC, as these hot rod guys are always sourcing parts for better brakes on their build ups.

However it ends up, be prepared to install new brake lines. Read up on how to re flair the brake line ends, bend up brake lines, etc. YouTube is your friend, along with videos from Eastwood.com