Mystery brake problem

[nsanow]

All,

I have a 1971 240Z that has a mystery brake illness. Here is what has happend and what has been done. I bought this car a couple months ago. Had a Datsun mechanic go through it. I had a leaking brake master and front caliper with black break fluid.

We have gone through 3 master's and I have new calipers in the front and I still do not have very good brake pedal pressure. When we are bleeding them on the lift the brake feels firm but as soon as I start the engine the pedal fades and nearly goes to the floor. I still have breaks but not near what they were before we changed the master. Here is what we have done:

New Sanyco master.

Yes the bleeder valves on the calipers are on the right position on top.

Yes we have bled the master cylinder as well as the lines in the proper order with the rear passenger tire first.

Yes we are nearly confident we have gotten all of the air out. We bled each tire extensively, waited a few minutes and bled them again until no air at all came out.

The last time we switched the brake lines on the master since the original master had the larger reservoir which I think helped some but it still feels soft.

I know I have to adjust the rear brakes since my e brake catches at the very top notch but the front brakes felt much stronger before we changed the original master.

Our only other guess is changing out the brake proportioning valve since the new master is aftermarket and used on Z's up to 1977 and maybe isn't set up right? This valve is directly below the master and has s lines going in and 3 coming out I believe.

Neither of us can figure out why my brakes are so soft. Any help would be awesome!

[sblake01]

Yes we have bled the master cylinder as well as the lines in the proper order with the rear passenger tire first.

This may or may not help but on an S30, the drivers side rear is actually the longest line and should be bled first.

[xray]

Is the reaction disk still in place inside the booster? If this little rubber disc falls out, pushrod travel will be limited inside the BMC and give the sensation of soft brakes, or brakes that don't engage until the very bottom of pedal travel. It sounds like the booster holds vacuum (given your comment about "softness" after the engine is started), but it might be worth hooking a vacuum gauge to it to be sure it still holds vacuum for a reasonable time period.

It's not uncommon for these disks to fall out/dislodge when changing master cylinders. Mine was displaced when the PO changed the master cylinder prior to my purchase.

A bit of a SWAG, I know, but if all else fails....

[nsanow]

xray,

We didn't look in the vacuum so that sounds like what may have happened. I won't have a chance to get into it until Friday or Saturday. But I thought of the booster was out I would have a really firm pedal and not such a soft, go to the floor pedal.

[Mat M]

I second sblake01's post.

Do you have old soft lines? getting stainless lines may improve the brake "feel".

[Babalouie]

That happened to me too, exactly the same symptoms. This is the reaction disc that xray's talking about:

It's the striped #18 thing in this diagram.

It's a 6mm thick rubber pad that goes behind the plunger here as it fits into the booster.

And you can see that it's a round rubber pad sitting in a little well inside the booster in this pic.

Remove the master, and feel inside the booster with your finger, that's what I did and hey presto...there was a little rubber disc resting at the bottom. Replacing it made the pedal nice and firm again but you might have to take the booster off to do it.

[nsanow]

Ok I finally got back in there this morning and have solved the issue. The first thing is there were still some air bubbles and we bled it more and more. We took apart the brake vacuum master but the rings were still intact. The issue was the pin needed to be lengthened. Once we lengthened the pin by 1/4 inch we have a taller pedal and better brakes. I still need to adjust the rear brakes since the e brake catches on the final notch so I know that will firm them up. The pedal still do not feel quite as firm as with the original master but I think that is because the new master is a generic aftermarket.

Thanks for your help and I am glad to finally have the car back on the road without sweating about me cracking it up!