What if I don't change my Master Cylinder on a disc conversion?

BillGrissom said
AJ/FormS is correct, and Physics works the same here in the U.S.

66fs poses a totally different problem:
If a clueless customer buys a new car and doesn't take it a brake shop until hearing metal scaping metal when braking, how big a reservoir is required so the inital fill of front brake fluid doesn't run out?

Of course, later cars added brake fluid level sensors, so they shouldn't reach such limits.

Actually, the flip-side is probably more important:
If during a brake job, the shop presses the caliper pistons back in, how large should an almost empty reservoir be so the brake fluid doesn't over-flow and ruin the firewall paint?

I like math too. But in actuality, AJ/S has made some incorrect assumptions that threw off his calculations from reality and led to the wrong conclusions. I just actually measured the drum-drum MC that the OP has for his '70 Valiant; the reservoir for the front brakes has a USABLE fluid capacity of approximately 50 cc's (NOT 140 cc's). That usable capacity starts with the fluid about 3/16" from the top of the MC reservoir down to where there is about 1/4" left above the ports in the MC bottom. That only gives you about 0.6" of fluid drop in the the drum-drum MC. (And the front brake's reservoir dimensions on this drum-drum MC are 2.5" long (average) and 2" wide if you want to check the numbers.)

What many are missing is that the drum-drum MC is MUCH shorter than the later disc-drum MC, so both of the reservoirs' fluid capacity are not all that ample. It is fine for drums where the adjusters take out the shoe wear, but not for caliper pad wear where the fluid has to fill in the long-term piston movement.

With a total pad wear of 0.7" of material per caliper (that's the total for both pads in a caliper), and assuming a 2" piston diameter (I don't have one to measure), the front calipers will suck out a bit over 70 cc's of fluid from new to worn down (where the pads are still NOT eating into the rotors). So yeah, it is pretty clear that the drum-drum MC will indeed run out of fluid in the front brake's reservoir well before the pads are worn out when it is used for front discs.

The design of the taller disc-drum MC and the longer front reservior when disc were introduced was done for good reason, as 66fs correctly posted.

The OP should change the MC IMO when going to discs, or be ready to add fluid regularly and then remember to slowly push back caliper pistons and suck out the excess fluid when renewing pads.