Rear disc brake master cylinder needed?

[srfjay]

So I am doing rear 240sx calpers in the rear of a 1970 with the stock 3/4 master cylinder. Some have suggested needing a bigger master cylinder like the 1 inch wilwood. But I have also read that a 1 inch is too big and the pedal will be too stiff.
Does anybody have real world experience with this? What masters have you used when going to rear disc?

[JT68]

Yes, we have done rear discs in the past. Most rear caliper piston sizes are small and not
a problem for the stock 3/4 master.

Bigger master cylinders are REALLY, really dumb. They DECREASE hydraulic pressure (100’s
of pounds if you do the math) requiring much more leg effort and flexing/breaking the firewall. Bad plan.
You don’t want a 7/8 or 1” cylinder. Those are for cars with power assist!

There is also a residual pressure valve in the rear master circuit which needs to be eliminated.
It was intended for drum brakes with spring returns. With discs it will cause the rears to drag or
possibly lock inappropriately. No bueno.

Not sure why you want rear discs, the big AL drums are more than adequate….

[srfjay]

Thanks for the info. I had most of the parts for the rear swap. It is more about newer parts and easier maintenance plus replacement parts can be bought at any auto store for a fraction of the price.
I did not know about that residual pressure valve. I will have to do some research on that. Maybe that is a reason people just replace it.

[srfjay]

Not a lot came up on that residual pressure valve. Would you happen to have more info on how to remove? Thanks

[redroadster]

Disk brakes do not have the braking ability /stopping power of drum.brakes and in the Roadster variant likely heavier
They do not overheats like drums in a severe use, but the alloy drums help considerably

[srfjay]

So you're saying drums are more efficient then disk? I have not heard that. From most test I have seen disk is superior. Hence why most manufacturers have switched to disk. Here is a clip from when motortrend did some testing.

"Disc brakes offer several advantages over drum brakes. Since the friction surface (rotor) is exposed, disc brakes dissipate heat more efficiently than drum brakes. There is also no problem with the brake's friction surface deflecting, as there is a pad pushing on each side of the rotor. A drum brake can deflect under extreme pressure, delivering less braking force and erratic stopping. Disc brakes do not retain water as do drum brakes. Disc brakes are also lighter in weight, easier to service, and brake more consistently than drum brakes"

[redroadster]

True but they stop better ....my point
which can lock
The big advantage to drums was rotors way cheaper to replace like said you can hot spot a drum if like racing it
&That needs replaced , but in my years on Datsun I rarely if ever had a alloy drum hot / dull spot
The Chevy ll drum brakes made you wear a seat belt ,or your head was wedged between the dash & glass Bias plys may have helped

[JT68]

Yeah, there is little advantage to swapping the rears to disc. (the one advantage is not having to adjust the rear shoes, but that is trivial and isn't worth the cost certainly) As point of reference, most every roadster race car ran rear drums due to the class rules. Those cars ran much faster than street cars and stopped fine. You just need a good working rear drum system.

Most folks don't even realize the residual valves are even there. The residual pressure valve is part of the check valve system in the outlet of each master cylinder circuit. The check valves allow the system to "pump up" allowing one way flow to keep a firm pedal feel. With the dual master, the front circuit has several bleed holes which allow more fluid to transfer and the fluid pressure to return to zero more quickly after the pedal is released. The rear circuit usually just has one small bleed hole. (the small hole allows slow pressure bleed while the spring and seal prevent macro low pressure reverse flow) Higher pressure/volume fluid return overwhelms the little spring. Parts 28,29,30 in the diagram.

In any event, if you still are going to do the rear conversion, you probably would want to replace the rear check valve with another from the front (making them both the same) Then add an adjustable proportioning valve because you want to prevent the rears from locking first and putting you in a spin. It's not really a great idea to experiment with street car brakes on public roads.

[JT68]

So you're saying drums are more efficient then disk? I have not heard that. From most test I have seen disk is superior. Hence why most manufacturers have switched to disk. Here is a clip from when motortrend did some testing.

"Disc brakes offer several advantages over drum brakes. Since the friction surface (rotor) is exposed, disc brakes dissipate heat more efficiently than drum brakes. There is also no problem with the brake's friction surface deflecting, as there is a pad pushing on each side of the rotor. A drum brake can deflect under extreme pressure, delivering less braking force and erratic stopping. Disc brakes do not retain water as do drum brakes. Disc brakes are also lighter in weight, easier to service, and brake more consistently than drum brakes"

Note these are hypothetical advantages and definitely not directly applicable to roadster rear brakes: Thin solid discs don't dissipate heat very well (vented rotors do), but AL is a much better heat transfer material.....Yes, drums might deform in extreme load situations (semis, dump trucks etc), but its a non issue on roadies, the AL drums are more than adequate...Retaining water? Might be an issue if you are doing river crossings with a 4x4, otherwise not so much....easier to service? well some are, others are not, the roadster rears are about as simple as brakes get. The roadster rear AL drum assembly is very light, probably lighter than the disc, caliper and bracket assembly you would replace it with. so as mentioned, really very little need for rear discs on roadster.....

[srfjay]

Ok that makes sense. I already had the calipers and disk. So in all it will cost me around 200 to switch to disk. Not counting I have new shoes and the aluminum drums plus some cylinders to sale. So it would be a wash on price. But I understand that the drums would be fine in most situations. But I also know these parts are getting harder to find and very pricey. So going disk i think would "future proof" the system .
And thanks for that diagram. Definitely helps.