Half Brakes or No Brakes

[540Z]

Hi Folks,
I have a rather large problem with the brakes on my '72 240z. I'm mostly finished with a resto/rebuild of a Scarab Kit 240z. I decided to replace the entire brake system with all new or N.O.S. parts. I have bled the system completely to remove all air, and even resorted to having a local garage power bleed the system for me. It appears that I still have minimal rear brakes. I only have half a pedal, and this diminishes when the engine is running. This is a completely stock brake system. I really need help with the following questions:

1. Are 7/8" Tokico and 7/8" Nabco stock Master Cylinders interchangeable?

2. What type of adjustments, if any, need to be made to my new vacuum
booster push rods?

3. Will a brake pressure switch from an early '77 280z work in the early '72
240z brake system? Does that switch also double as a proportioning valve?
Could this be the cause of the trouble?

4. How do I know if my rear proportioning valve is working? At this time, I
can rotate the rear wheels by hand if the brakes are applied.

Any help or advice you all can render would be greatly appreciated.

Thanks again,
Tony Russo

[dogma420]

....completely stock brake system....resto/rebuild of a Scarab Kit 240z

What does this mean? They are 2 different things...

Never heard of the stock 'rear proportioning valve' malfunctioning...I've been told it's not a real one...could be wrong though. If the rear brakes are bled correctly, it sounds like your cylinders are defective...have you taken off the drums and just watched the cylinders expand with someone else pushing on the pedal? How does the pedal feel when you press on it? Not enough info.

[540Z]

The brake system is entirely original spec for the car. The Scarab Kit stuff applies only to the drive line (i.e., engine mounts, motor, trans, radiator, frt sway bar, drive shaft, shifter only). As far as the cylinders expanding, they must be a little as they are applying some, albeit slight pressure onto the drums from the brake shoes.

[540Z]

I neglected to completely answer your question, Dave. The pedal feel is light to about the halfway point, then spongy, pressing almost to the floor. The pedal feels firmer when the engine is off and there is no vacuum applied to the booster. I will try to observe how much brake shoe/wheel cylinder travel exists.
I know the parking brake is only holding near the very top of its travel, and I've ran the adjusters up some to try to close the distance between shoes and the drums without much success. The drums seem to be within their wear limits also. What gets me is the fact that all these parts are new or N.O.S. Thanks for your help,
T.R.

[datto_240z]

If they are the drums that came from the factory 30 odd ears ago, the chances are that they are rooted, also are your brake shues new or used? It sounds as though they are not ajusted properly. with the drum on you should be able to turn the drum but feel a bit of resistance. If they are ajusted correctly, it could be the drum/shue size is above min/max size.



Chris

[dogma420]

have the rear brakes been adjusted yet? Do you have the adjusters (inside between the shoes) installed correctly? Are you sure?

Adjust the rear brakes by applying the parking brake over and over about 8 times.

Sounds like you have the adjusters installed incorrectly, possibly. That is good news because it is real easy to fix, (and real easy to do in the first place).

[mdbrandy]

Just as a thought, search for the term "reaction disk" in the forums. I've seen a couple people complain about similar problems, and the difficulty was that they'd let the reaction disk in the vacuum booster fall out during assembly.

[540Z]

Thanks guys,
I am currently investigating the mechanical aspect of the rear brakes. It makes sense that the drums could be out of spec.. I assembled the rear wheel cylinders to the vehicle per the manual but it mentioned to set the shoe adjustment fairly close and let the parking brake mechanism take up the slack by ratcheting it up several times. Apparently that didn't work. I can still push the vehicle with the parking brakes set, so I'll keep trying.
As for the reaction disc in the vacuum booster, what is its function, location in the booster, and appearance. Nothing fell out while I was assembling it to the vehicle and it holds vacuum when applied.

[mdbrandy]

Apparently that didn't work. I can still push the vehicle with the parking brakes set, so I'll keep trying.
As for the reaction disc in the vacuum booster, what is its function, location in the booster, and appearance. Nothing fell out while I was assembling it to the vehicle and it holds vacuum when applied.

If the parking brake doesn't pull enough to engage the rear brakes, that would seem to be a problem, so you're probably looking in the right place.

As to the reaction disk, it is inside the booster, and apparently it falls down inside the booster sometimes if you aren't careful when reattaching the master cylinder pushrod. It has never happened to me, and I've never looked, so all I know is what the other threads have said. Search and read if you run out of other ideas.