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%

[cooldill]

I was hoping to piggy back on this since I am running into issues running this same setup. Rebuilt Zx fronts with stock rear drums, 7/8 zx master. 
 

Rears bleed out fine but don’t have any fluid flow through the fronts. I get fluid flow through the hard lines but not enough to push through the soft brake lines. 

there is about an inch of brake feel at the bottom before the floor. 

I have also bench bled the master and have deduced that the stock push rod is too short for this setup, I have it fully extended and still the same result. 

 

 is there something I’m missing here? how do I know how long to extend the rod?  I was hoping to get an off the shelf adjustable rod to replace the stock one. What size should I look for? 
 

thanks in advance 

 

[Icehouse]

I've ran my cars with zx up front and stock rears.  Doesn't really matter what master, they both just feel like manual brakes.  Worst I ever did was giant master with stock brakes.  It basically had anti lock brakes.  Made it really hard to jump into a modern car haha.  It was my daily for 5 or 6 years like that.  My autux car also if you follow any of my projects.  

If you want to run the stock style master rod on a master meant for a booster you need to pull the piston and shave it down the thickness of the retaining washer for the rod.  You may also need to carve out the center hole a hair too.  This allows the piston to move back far enough to release pressure.  I put tape over the piston so grinding dust doesn't get in the seal.  

[Stoffregen Motorsports]

13 hours ago, Icehouse said:

If you want to run the stock style master rod on a master meant for a booster you need to pull the piston and shave it down the thickness of the retaining washer for the rod.  You may also need to carve out the center hole a hair too.  This allows the piston to move back far enough to release pressure.  I put tape over the piston so grinding dust doesn't get in the seal.  

I was always under the impression that if the piston did not return all the way, ie - stuffing the pedal rod in there without massaging the washer, that the brakes would pack up and not release. So in that case, it either works or it doesn't work. @cooldill - do the brakes stick on?

 

Also, I've seen it a dozen times or more, people connect the master incorrectly. The front reservoir on the master is for the rear brakes, and vice versa. Heck, I've even done it myself, and the vehicle in question never had fluid flow at the front calipers. That's what made me think of it.

[Draker]

I’ll go look at mine and report back. Mine stops great and feels pretty good.

[Draker]

I’m running a 7/8 master. I believe I removed the residual valve for the front also. 

[Dguy210]

As another data point I'm running a 3/4 master on my B210 with 280zx fronts and stock backs (interchangeable with 73 510) and a brake booster. All the rest of the system is stock and I have stock drums in the rear (from a 78). My pedal feels fine to me (likely a bit mushy in reality) but functions just fine. 

[datzenmike]

Some find the longer travel easier to modulate and hold near lock-up.

[DHale_510]

The longer travel is easier to modulate but it feels best on a second pump. Not easy to recommend this to someone I don't know. I also relocated the hole in the pedal lower, the original hole was worn anyway. The stock 240Z master was 11/16, right in the middle and is worth the effort to make work without the booster. The big ZX ones always want the booster that is usually too big in a 510. There were some small boosters on 10s and maybe some pickups that I have used with SUs but not Mikunis.

[Icehouse]

I forgot to mention my datsun 1200 has ZX brakes and a stock master.  Pedal feel is great and stops awesome.  

[Draker]

47 minutes ago, Icehouse said:

I forgot to mention my datsun 1200 has ZX brakes and a stock master.  Pedal feel is great and stops awesome.  

 

Yep same. My 1200 has them too, stock master, didn't pull the residual valve. Works great!

[cooldill]

On 8/14/2024 at 7:47 AM, Stoffregen Motorsports said:

I was always under the impression that if the piston did not return all the way, ie - stuffing the pedal rod in there without massaging the washer, that the brakes would pack up and not release. So in that case, it either works or it doesn't work. @cooldill - do the brakes stick on?

 

Also, I've seen it a dozen times or more, people connect the master incorrectly. The front reservoir on the master is for the rear brakes, and vice versa. Heck, I've even done it myself, and the vehicle in question never had fluid flow at the front calipers. That's what made me think of it.

 

 

thank you for all the replies. This is my first project like this, so very much a newb. For some reason I was thinking that the second reservoir was a backup, rebled master at both tanks using a plastic tube from the bleeder back into the tank and then bled the front brakes. now it finally bleeds! Thank you for the help gents. 

Trying to get this project on the road while caring for a newborn is crazy, but I appreciate the help, saved me and my baby a bunch of headache.