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seattlejester

seattlejester's 1971 240Z

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Goodness I love this forum, you really can find everything.

 

Quote from this thread:

DO NOT, DO NOT, DO NOT preload the brakes. EVER!!!

 

The master cylinder needs to fully release in order to open up the little ports that let the brake fluid back into the reservoir. If they can't release ALL THE WAY then they can't release the pressure.

 

This makes no difference at first, because you let off the brake and there is no pressure, then you step on them and there is, let off and there isn't. As the brakes gets hotter from you applying the brakes, say, in traffic, the heat generated transfers into the fluid, which expands. If the pressure can't release back into the master cylinder reservoir then this results in a light drag on the brakes. As the drag continues, more heat is generated, more fluid expansion, more pressure, more heat, more expansion, more pressure, etc.

 

I had this happen with my clutch once and I learned to have free play in the system. On the brakes you want a little free play in the master to booster connection, and you want a little in the pedal to the booster connection. 1/8" of free play in both won't make a noticeable difference in the pedal throw, but not having that free play will be a BIG problem.

 

Pretty sure that is what happened. 

 

I measured what I did and found that I was sticking 13mm past the flange of the aluminum spacer at first, that caused the pre-load on the master and the exact symptom of the brakes coming on and staying on. Then on the side of the freeway I moved it down to 6mm past the flange and the car drove back with no problems. I have it currently set to 9mm which is in spec, but I will be checking for preload and complete release to make sure I don't run into problems again. 

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