Datsun Roadster Forum-311s.org

Could someone educate me, possible with pics, of the differences between a stock Roadster brake master cylinder and ones from a 510 and a 240/60/80Z? My car currently has the stock master and is about to get an upgrade.

Am I correct that the stock master is 3/4"? Some were 13/16"?
The 510 is 3/4".
The 240/60/80Z is 7/8?

Could someone who has swapped in one from a 240/60/80Z explain to me (ELI5) what modifications/accommodations had to be made for that swap? Pics?

My car has Spriso's 300ZX front brake conversion and a rear from an RX-7 (rear disc). It stops well enough right now, but the pedal is softer than I like and travels farther than I like. It makes it difficult to heel/toe.

My 68 Roadster was equipped with a 3/4" master cylinder. I replaced it with a 510 3/4" master cylinder. The only modification required was installing two brass connectors/adapters between the master cylinder and the original brake lines. I bought the connectors from a NAPA store where they were readily available.

The front and rear brakes are original discs and drums.

The softness is probably due to air in the line. A larger bore won't help with that, but it may reduce pedal travel.
I believe my Z had a 7/8", and I changed it to 15/16". I can't say I noticed much difference in the travel. However, the Z has vacuum assist, so the brake pedal is a bit more vague than the roadster.

You might ask Karps if they can overbore the stock master to a larger diameter while resleeving. Of course you'd need a larger piston as well, but when you look at the price that some charge for new roadster MC's, the cost may be less than buying a new master.

Brian Zana/Sin City Datsun has new aftermarket master cylinders at a very attractive price.

If you increase the size of the master cylinder then you will have to exert more pressure on the pedal for the same amount of stopping power. Since you have changed brakes at both ends the stock master may not be sized correctly. I would check with Spriso for their advice since they engineered the swap.

theunz wrote: Tue Jan 12, 2021 2:18 pm If you increase the size of the master cylinder then you will have to exert more pressure on the pedal for the same amount of stopping power. Since you have changed brakes at both ends the stock master may not be sized correctly. I would check with Spriso for their advice since they engineered the swap.

Yes, Michael has said he recommends a 7/8" master for my setup. I know one from a 240/60/80Z doesn't quite 'just drop in' and so I am trying to get a solid idea of just what is involved with that swap. Something about a washer to change travel and a different rod?

As I remember, I used the 7/8" Z master. Took an adjustable rod from a spare Roadster Clutch Master. Be sure the end of the rod is the same profile as the receiver cup on the piston of the Z master. Then "machined" a bevel on 1/4" x 1"" fender washer after "machining" the O.D. down to fit the 7/8" bore. The machining was done in my cordless drill with a file. I then assembled the unit and adjusted the length of the rod to compare with my stock 3/4" master. Hope the pics help.
Pat

dynaguy wrote: Tue Jan 12, 2021 5:24 pm As I remember, I used the 7/8" Z master. Took an adjustable rod from a spare Roadster Clutch Master. Be sure the end of the rod is the same profile as the receiver cup on the piston of the Z master. Then "machined" a bevel on 1/4" x 1"" fender washer after "machining" the O.D. down to fit the 7/8" bore. The machining was done in my cordless drill with a file. I then assembled the unit and adjusted the length of the rod to compare with my stock 3/4" master. Hope the pics help.
Pat

Pat, thank you very much.
Please pardon my ignorance, but is the machined washer 'inside' the rubber gasket in the photos? The 'washer' is on the inside end of the clevis rod and is what is pushing down thru the bore?

I have what i recall is a band new 7/8 master without reservoirs available, as I had considered going to the larger master given my brake mods but decided to not make the change. If you're interested I'll sell it for less than the best price on rockauto for the equivalent master plus shipping as it's of no use to me. Let me know, I can send pics.

onsight512 wrote: Tue Jan 12, 2021 5:57 pm

dynaguy wrote: Tue Jan 12, 2021 5:24 pm As I remember, I used the 7/8" Z master. Took an adjustable rod from a spare Roadster Clutch Master. Be sure the end of the rod is the same profile as the receiver cup on the piston of the Z master. Then "machined" a bevel on 1/4" x 1"" fender washer after "machining" the O.D. down to fit the 7/8" bore. The machining was done in my cordless drill with a file. I then assembled the unit and adjusted the length of the rod to compare with my stock 3/4" master. Hope the pics help.
Pat

Pat, thank you very much.
Please pardon my ignorance, but is the machined washer 'inside' the rubber gasket in the photos? The 'washer' is on the inside end of the clevis rod and is what is pushing down thru the bore?

Unscrew the yoke off the rod, slide the washer, beveled side down, onto the rod. This will mate to the bevel in the master bore. The washer will stop at the "mushroomed" end of the rod that matches up to the cup on the master piston. There is a cir-clip that goes into a groove in the housing that holds everything together. Slide the rubber boot over the rod, re-install the yoke, adjust for length using locking nut. You're good to go.

Pat

dynaguy wrote: Wed Jan 13, 2021 11:46 am

onsight512 wrote: Tue Jan 12, 2021 5:57 pm

dynaguy wrote: Tue Jan 12, 2021 5:24 pm As I remember, I used the 7/8" Z master. Took an adjustable rod from a spare Roadster Clutch Master. Be sure the end of the rod is the same profile as the receiver cup on the piston of the Z master. Then "machined" a bevel on 1/4" x 1"" fender washer after "machining" the O.D. down to fit the 7/8" bore. The machining was done in my cordless drill with a file. I then assembled the unit and adjusted the length of the rod to compare with my stock 3/4" master. Hope the pics help.
Pat

Pat, thank you very much.
Please pardon my ignorance, but is the machined washer 'inside' the rubber gasket in the photos? The 'washer' is on the inside end of the clevis rod and is what is pushing down thru the bore?

Unscrew the yoke off the rod, slide the washer, beveled side down, onto the rod. This will mate to the bevel in the master bore. The washer will stop at the "mushroomed" end of the rod that matches up to the cup on the master piston. There is a cir-clip that goes into a groove in the housing that holds everything together. Slide the rubber boot over the rod, re-install the yoke, adjust for length using locking nut. You're good to go.

Pat

Thanks Pat!