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Brakes locking up. Need Help!!


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#1 murdrdatsoon72

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Posted 12 June 2018 - 12:43 AM

I have the usual 280zx front strut and disc brake setup on my 72 wagon. My issue is when I first drive off the brake pedal operation is soft. After about 20 minutes of normal driving the brakes start locking up and the brake pedal gets really hard. I rebuilt the front calipers  (sand/polished piston and bore) new seals and bled Master. I thought at first it was just the front brakes but when I checked the rears (drums) they too were locking up. If I let the car cool down, the brake pedal is soft again until I start driving it for awhile then they lock up. I am running the 280zx dual Master and just picked up a porportioning valve from a 81 280zx. The problem with this setup is my original setup has 5 lines going into it plus brake switch and the 81 280zx only has 4 and no brake light switch.

I was told that the wagon does not have a porportioning valve rather a hydraulic pressure switch in the brake system.

Can anybody give me some suggestions on this problem?



#2 Crashtd420

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Posted 12 June 2018 - 02:11 AM

Only thing that pops into my head is are you over full in the reservoir?

I had an atv do it to me, because it was to full it heated up and it locked the rear tire....

In my case I cracked the bleeder to relieve the extra pressure and everything worked good, later drained some fluid from the reservoir...
And when I did crack the bleeder fluid shot under some force so be careful...

#3 mklotz70

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Posted 12 June 2018 - 03:56 AM

Did you mess with the push rod at all?  It sounds like your m/c's piston is not fully returning to where it should.  It can either be the push rod is adjusted too tight or you have a bad m/c.  When it sits, the fluid probably seeps back into the reservoir.

 

Do your self a favor and get a switch that works on the brake pedal.  That way you get brake lights as soon as you touch the pedal instead of after the pressure gets high enough.  You have to extend the wires into the cabin, but it's definitely worth it.  Hmmm.....I see it's a 510.....I thought they already had the pedal switch.  If the switch in the engine compartment has only one wire, it's a pressure differential switch that turns on the ebrake light on the dash when one of the circuits loses fluid/leaks.


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#4 banzai510(hainz)

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Posted 12 June 2018 - 07:53 AM

I had a bad Master that would pump up everytime I hit the brake and not release the pressure unless I hit the blled screw to release it. It was all 4 wheels.

 

It might be your proplem or not.


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#5 Dolomite

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Posted 12 June 2018 - 09:24 AM

I’m with mklotz on this. You need about 1/8” of play between the rod end and the piston in the master. I just went through this after getting my new 510. I put in a bit of play and now everything is kosher.
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#6 murdrdatsoon72

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Posted 12 June 2018 - 11:43 AM

Thank you all for your advise. I will check these suggestions. I did not adjust the brake play, thought about it. Has anyone installed a 280zx proportioning valve on their wagons or should I just keep the original pressure switch? The 280zx MC was bought new about a year ago. Car normally sits in garage (3 years now) because its not street legal yet. Also this braking problem needs to be resolved.



#7 datzenmike

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Posted 12 June 2018 - 11:51 AM

I'm not convinced there is a proportioning valve. Just the brake warning light. The proportioning or 'bias' is easily built into the brake system by engineering the sizes of the calipers on the back to complement the fronts. Simply put.... why have rear calipers that are too large to begin with that need a proportioning valve????

 

 

Also the rear system should have a drum brake residual valve in the MC. If a 280zx, then you will have a disc brake residual valve in there..


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#8 murdrdatsoon72

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Posted 13 June 2018 - 09:33 PM

Problem solved!! Mklotz and Dolomite was right. Thanks Bros!! The pushrod was too far extended, I backed down the rod to 1/8" and the brakes work perfect without locking up!! 

 

Anybody interested in an 81 280zx proportioning valve (used) with all connectors and clamps? :thumbup: Just bought it off Ebay!



#9 mklotz70

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Posted 13 June 2018 - 10:29 PM

Glad to hear you got it figured out. :)


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