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Ever since I got my 78 Z the brakes have felt terrible. At first it was the reactionary disc causing my woes, and I replaced my booster with a reman unit that had the disc inside, mine was unfortunately long gone and not rattling around inside my booster. After replacing the booster the brakes definitely felt way better, it was no longer an on/off switch that would throw my face into the steering wheel, but the pedal still felt off and I couldn't bleed all the air out of the system. I ended up tracing this to the front passenger caliper having an internal leak and replaced it, but after bleeding everything three times my pedal is still spongy and doesn't engage any brakes until about 1/4-1/2 way down the pedal travel. I can pump the pedal up a bit, which to me indicates there's still air in the lines, but I don't see any air coming from the bleeders. I'm pretty sure I have my process down correctly, I start at the master cylinder and then move to the rear driver's side and then go counter clockwise around the car from there, but no matter how many times I bleed no more air comes out and the pedal still feels exactly the same.
 

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8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
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Are your rear brakes adjusted?
If everything is free and not rusted, the hand brake should keep them adjusted.
But it doesn’t take much to keep the adjusters from working.
 

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Are you using DOT 5? I've heard it spongier than 3 or 4.
Some things mentioned in the FSM that can cause a spongy pedal (other than air); clogged reservoir filler cap vent hole, deteriorated rubber brake lines (they bulge when brakes are pressed), bulging brake lines, re-using brake fluid or poor quality brake fluid.

It sounds like your brakes begin to engage once you've depressed the pedal 1/4 to 1/2 way down. Have you adjusted the pedal? Instructions are in the FSM. Do you have a larger than stock master cylider? If so, the pushrod might be the wrong length or need adjustment.

Lastly, have you tried reverse bleeding the brakes? Air wants to travel up in fluid, might as well take advantage of this rather than trying to force it down.

Oh, bleeding order should be: rear passenger, rear driver, front passenger, front driver.
 

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Another one from the FSM for excessive peddle travel is faulty check valve.

I’m wondering if there may be air in the master cylinder though.

Sent from my iPhone using Tapatalk
 

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Yes, it is possible that the source of the air that you are experiencing based on your description, is a result of air 'trapped' in the master cylinder.
Bench bleeding the master is a good troubleshooting pathway that will eliminate master cylinder air.
However, there is a quicker and simpler troubleshooting trick I'd try first; Apply a generous amount of axle grease to all the brake fittings, preferably one at a time. The heavy grease will act as a seal from any air being sucked into the brake line fittings / joints. If you are patient, and do one fitting connections at a time, you can narrow the problem down.
Its a quick trick that has worked for my 1978 280z and other older rides.

Let us know what finally works.
 
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