510 w/ 280zx front and stock rear drum

[DARIN 510]

I know there’s plenty of info on the Dime quarterly discussing how to do the swap but that’s for 4 wheel disc. I had a question in regards to running 280 ZX front struts with stock rear drums on a 510. I’ve seen people talk about using the 7/8 master cylinder And also stock 3/4 master cylinder. But ideally, which direction would be best for street use? I have both spares kicking around here, but they need rebuild or replace. Trying to do this on the cheap. any guidance is appreciated. 

[datzenmike]

The zx uses a 15/16" master to provide enough volume of fluid for the massive front disc brakes. Smaller diameter has to travel further to do the same work and the pedal traveling this much feels mushy. Still works but having the pedal so close to the floor is unsettling. Some say the longer travel makes breaking right at the edge of wheel lock-up easier.

 

Increasing the diameter from stock means more fluid moved and as this invokes the "you can't get something for nothing" law the pedal will feel very hard to operate. Best fix is to add a brake booster. After late 60s all Datsuns have boosters because the brakes got larger and better and the cars a LOT heavier and needed the extra help. The B-210 or the 620 booster is small enough to fit above the steering column below and the clutch master to it's left. 610 and 710 might work also.

[DARIN 510]

Thanks Mike. But I don’t plan on running a booster. On my white 510 I am running the 15/16 master with 4 wheel disc . So what would you recommend to run 15/16 on stock rear drums?

[datzenmike]

It's the much larger front calipers that use up all the pedal. The rear drums don't matter much. The 3/4 will work but may feel soft and mushy. If you go up in size the braking effort increases that's all. Just something to be aware of.

 

Also the zx 15/16" has a disc brake residual valve in the rear circuit and this should be replaced with a drum brake valve. The residual valve retains a small amount of brake line pressure. Disc brakes, as on the fronts, only need about 2 PSI, just enough to keep the pads lightly against the rotor to keep them 'clean'. Rear drums need about 4 X that amount to remove all the slack in the shoes and return springs.  I put a zx 15/16" on my 710 and just used the 710 rear residual valve in it for my stock drum brakes. The 710 has a brake booster so I didn't notice any difference in effort. Brake boosters increase brake line pressure output by about 50%