Brakes work but pedal is soft

[Cooper260z]

I’ve done some looking but haven’t found anyone describing this particular scenario. 

My car is an early 74 260z, I replaced the rotors, pads, rebuilt the calipers, new drums, shoes and hardware as well as new wheel cylinders and master cylinder. 
The brakes work pretty well though they don’t lock up the wheels but they don’t have very many miles on them so I don’t expect them to yet. 
 

The brakes work but the issue is that the pedal has nearly no resistance when depressing it while the car is running, it just goes to the floor. The braking force increases the further the pedal travels, the pedal just doesn’t get harder to press like you’d expect. 
 

The only thing that I can think of is maybe I don’t have the correct master cylinder? I bought it off rock auto rather then a Datsun oriented site. The front fluid reservoir feeds the rear brakes and the rear reservoir feeds the front and the master cylinder has “⅞” cast into it. So maybe it’s the wrong size?

 

The car is very drivable as is but isn’t very confidence inspiring like I’d prefer it to be when it eventually hits the backroads. 
 

Thanks in advance! 
 

[Terrapin Z]

One thing that happens is the front calipers can get installed on the opposite side of the car from where they came from. 

You can double check this by looking at the bleeder, It needs to be at the top of the caliper. 

When the calipers are on the opposite sides the bleeder ends up on the bottom making a proper bleed impossible. Makes for a sold pedal with the air still in there. 

 

[Cooper260z]

@Terrapin Z thank you, I hadn’t heard of that before. But I just checked and both bleed screws are on the top, so that’s not it. 

[heyitsrama]

I used a generic one from rock auto on mine, I prefer to use beck arnley as I’ve had good results with their products.

have you confirmed bleeding air from the system out? What do the rubber hoses look like from the hardline to the calipers?

[siteunseen]

The short hose from the hardlines to the wheels, what kind of shape are they in? I, as did most others, put stainless braided hoses there and got rid of the old rubber lines.

Heyitsrama said it better. Hardlines to calipers.

Edited December 8, 2020 by siteunseen

[heyitsrama]

@siteunseen

 

@Cooper260z the other thing that it could be is the pushrod that between the booster and the master cylinder that needs adjustment. But double check for air first. 

[Terrapin Z]

23 minutes ago, Cooper260z said:

@Terrapin Z thank you, I hadn’t heard of that before. But I just checked and both bleed screws are on the top, so that’s not it. 

Just ruling out some known issues. 

Another thing that often happens with the brake system. (it happened to me) is the reaction disc in the booster falls off the push rod for the M/C. It is a small rubber pad. Without it the brakes can act really erratic.

it is the dropped down rubber piece in this picture. If it is not in place it is likely inside the booster shell. 

After fishing mine out, I glued it on to the metal pad.

[Cooper260z]

The soft brake lines I also replaced. I replaced with new rubber lines because I didn’t have the budget for stainless at the time and to the best of my knowledge they’re bled properly, brake fluid comes out clean. And I’ve adjusted the push rod, that was another step that got to my current scenario. 
 

@Terrapin Z I’ve heard about that disc in the booster before, how would I check for that?

 

[Terrapin Z]

1 minute ago, Cooper260z said:

 I’ve heard about that disc in the booster before, how would I check for that?

It is a pain after you have assembled everything. You have to pull the M/C back off the booster and extract that rod. The pad sits on that flat metal part inside the booster. 

The rubber coated disc behind that metal head in the pic is the seal for the M/C opening and just slides out. 

No way of knowing until you pull it out and confirm the rubber pad is mounted on the metal head. 

Even if you did not pull the rod out of the booster, that pad can just fall off once the pressure is off of it. (pretty sure that's what happened on my car. I was just changing the M/C) That's why I JB weld'ed it on to the metal.

Rule everything else out that is suggested here, but it just might be the issue.

[Cooper260z]

if memory serves though, the rod won’t just come straight out of the booster, so I assume there’s some step that I’m missing to get it out. I’ll get out there soon but if you know what step I’m missing to get the rod out that would be great info. 
 

 

 

[Zed Head]

Did you adjust the rear brakes to where the shoes just barely drag on the drums?  Try pumping the parking brake handle and see if things get better.

Does the brake light come on when it goes to the floor?  That would show a pressure imbalance between front and back.

Does the pedal get higher and harder if you pump it?  That would be air in the system.  If it doesn't pump up it's probably mechanical, something needs adjustment.

Sometimes the new master cylinders are just bad.

Things for your list...

[Cooper260z]

Yes. No. No. Short answer, I can rule those things out. Didn’t know that about the brake light showing a pressure imbalance, that’s good to know