Mystery 71 240Z brake issue

[nsanow]

Okay guys I need some help as I have been chasing this issue around for nearly a year!! I have a mid 1971 240Z. I have replaced the master cylinder to a 7/8 aftermarket, both front calipers, disks, rear wheel cylinders, drums, and put in stainless steel brake lines. When I originally put in the master back in June 2008 we adjusted out the pin in the brake booster and switched the brake lines on the proportioning valve since that is what I read needed to be done with the aftermarket master.

I just took the car to a shop to have it professionally bled and worked over. It did still have a leak but now it is all tight and airless. The issue is that the brake pedal will go 2/3rd of the way down before the pin engages the master cylinder. The brakes then start to grab and I can stop but the pedal will go to the floor. This doesn't instill a lot of confidence in my high speed driving I would like to do.

The mechanic said he thought the pin was adjusted all the way out but I am not convinced. Any ideas? Could this be a bad master cylinder? I am thinking since we can hear it and feel it engage maybe not. Is there a longer pin for the brake booster? Any help would be great! Thanks

[=Enigma=]

Sounds like this might be due to a missing reaction disk. This part can fall into the brake booster when changing out the master cylinder. Also, switching the brake lines is only necessary when switching from an early style MC to a later style (not sure where the cut off was) and is supposed to happen at the MC, not the proportioning valve. Also, could you be confusing the safety switch with the proportioning valve? Note that the MC is clearly marked with F & R for front and rear. Use the search feature for more info on both issues. Here's a starter link for you:

http://www.classiczcars.com/forums/showthread.php?t=23401

Edited May 1, 2009 by =Enigma=

[nsanow]

That is a good idea and I will have it checked out. I should have been more clear we did the line swap at the master not on the proportioning valve. I do know the proportioning valve so that is good. How would the reaction disk shorten the pushrod to engage the master? Thanks for your help!

[=Enigma=]

The reaction disk fits between the rear end of the push rod and the power piston, contributing to the overall length of the push rod. Without this, your pedal throw before engagement will be longer. If this is the issue, you'll need to re-adjust your push rod length after the reaction disk is put back in place. See attached image of a 1973 booster for reference. This is a little differnet than yours but will work for illustration purposes.

If you haven't already, be sure to bleed your brakes when done. Based on the configuration of the piping on the car, start with the rear driver side, rear passenger side, front passenger side, and finally the front driver side.

1973 Brake Booster Diagram

Edited May 1, 2009 by =Enigma=

[kinfish]

The brake system should be set up with a small (.030-.050) gap between the brake pushrod and the piston in the master cylinder. If you have lotsa pedal travel and no resistance you are not moving the piston in the master cylinder.

You may be missing the reaction disc in booster as Enigma suggested or you are victim of mismatch due to different length adjustable pushrods. Pull the stuff apart and take some precise measurements to calculate what gap you have in free state. I have seen at least two pushrods of different overall lengths in parts supposedly good for '71's.

[Riwanika]

I have this exact same issue, and it has been driving me up the wall. Did you ever find a solution to your problem? Or was it a never ending issue? Please let me know, i want my Z back!

[240260280z]

Pull out the push rod from the brake booster and fish around the bottom of the can for the rubber reaction disc.

[Zedyone_kenobi]

Here is another option. When I replaced my brake booster, the adjustment pushrod that came with mine, was not long enough. Even all the way out I was short the correct distance by a good 1/8 inch. I had to buy a bolt with the correct threading and then grind off the end to a smooth ball before I could get enough length. But then it worked perfectly.