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Master cylinder leak out of cap.


Motschrocker

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So i just put on a new master cylinder for a 73 610. I bench bled it and bled the system once on. The pedal was solid and i had good stopping power. My problem now is that brake fluid from the rear resevoir of the MC is being pumped into the front resevoir and overflowing through the cap. It does this when i pump the pedal. What would cause this. Do i have a bad MC?

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You are under the assumption that because something is new it can't be the cause of a new problem. The opposite is true. The reservoir didn't over flow before the new part was added, but now it does. The new master is a total unknown quantity, worse and even more suspect is it may be a cheaply rebuilt (cheap parts and labor) or a cheaply manufactured knock-off from China. Both are not anywhere near to the original equipment quality demanded by Nissan. 

 

There is no proportioning valve on the 620 brakes.

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Well there are two systems inside the master sharing the same cylinder. They are separated by rubber seals. It's not a stretch to see that one could be forced into the other. It's been reported time and again the $19 knock offs are 50% bad in the box. They have been taken apart and found to have aluminum 'chips' from the machining process inside. Dollar an hour workers put part A onto part B and stuff into part C.

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  • 4 weeks later...

So i ordered another master cylinder and installed it. Different brand this time and it is doing the same thing. Fluid is taken from the front resevoir and overflowing out of the rear. Is it possible i got 2 bad mc or is there something else causing this. I can upload pics if that would help anyone

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I changed the original because fluid was leaking out next to the brake booster. The first mc did not move fluid from one resevoir to the other or at least not that i noticed. Once i saw the leak i replaced it. I will take some pics of the set up tomorrow and maybe that will spark some new ideas.

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You are under the assumption that because something is new it can't be the cause of a new problem. The opposite is true. The reservoir didn't over flow before the new part was added, but now it does. The new master is a total unknown quantity, worse and even more suspect is it may be a cheaply rebuilt (cheap parts and labor) or a cheaply manufactured knock-off from China. Both are not anywhere near to the original equipment quality demanded by Nissan. 

 

There is no proportioning valve on the 620 brakes.

This^^^^
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I am not ruling out that i could have gotten another bad mc, but i cant seem to find anyone who has experienced an issue similar to mine. I should mention too that the fluid only transfers from one reservoir to the other when i let up on the pedal and not when i push down.

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I haven't got any pics uploaded yet but let me ask another amateur question. My brake system has both a brake pressure warning switch and an NP-valve in the lines. Could a malfunction in either of these cause my problem. If not i have seen some new old stock MC on ebay. Would i have better luck with one of those?

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  • 4 months later...

Well i did not my car all winter but im back at it to solve my brake issue. I pulled NP-valve and took it apart. Gave it a good cleaning, but for the most part the internals seemped to be in good condition. I also took off the brake pressure switch. Took that apart and i could look straight through it and no valving or springs were in it. Im not sure if that is right. Maybe that is where i am having issues. So i am wondering if my best option is going to be a universal proportioning valve and just run new lines. I put some pictures in the following link.

https://picasaweb.google.com/103137401177043918465/66L18Roadster

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Pretty sure the 610 did not use a proportioning valve. The front to rear brake bias was designed into the system by using the correctr size rear wheel cylinders. This saves parts and time by the maker of the car.

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Early Datsuns used fixed proportion brakes, but in the 1970s some were fitted with a "Load Sensing Valve Assembly" called the NP-Valve. This allows good braking whether fully loaded or empty.

 

It is a proprtioning valve.

 

610/B210 NP-valve was integrated into the splitter/pressure differential switch.

 

In the 510, it replaced the 2-way connector which all 510s have.

* 46400-A1100 "P" VALVE ASS'Y (op.) DX

* 46400-A1100 ASS'Y-VALVE, proportioning (op.) S&C JDM

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Well i took apart both master cylinders. The original one that was on it is a nabco and it is is really cruddy on the inside. I compared it with the new one i had gotten and they look essentially the same as far as how the piston assembly is set up. I am not really sure what to do at this point. Rebuild the old one? Try another new one? Is it possible something else in the system is causing this.

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