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Brake bleeding


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Can’t get brakes to pump up. 

New booster, New MC, ...I bench bled the new MC (it pumped up hard) and installed. Started at the right rear, left rear, right front, and finished with the left front. Went around the truck 10 times and only get a tiny bit of peddle at the very bottom .   All bleeders spray clean clear fluid when released.  When all bleeders are tight ...u can pump the peddle 50 times and still only a tiny bit of peddle at the very bottom .  No leaks at all when pumping with bleeders closed . I bled the bottom two ports on the side of the M/C too and got good squirts.   Any ideas.  I know people say bleed,  bleed and when your done..bleed some more:) but I’ve never had a oroblem getting them to pump up in other vehicles on the 3 or 4th pump. The when cylinders I know nothing about where it has sat for a while, but I figured if I’m getting fluid at the wheels they seem to be working.  I figured I’d at least get some peddle ????






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Did you blow out the lines to make sure they weren't clogged?


Sometimes, to get the fluid running, I will open the bleeders (maybe just the rears or the fronts or all four) and slowly pump the pedal. Once it starts to flow, then you can resume your proper bleeding procedure.


I am assuming you have the master properly adjust at the pedal, with about 1/16" free play.


If you absolutely cannot get fluid running, you can cover the reservoir with a rag and blow low pressure air into the reservoir (with the bleeders open). If this doesn't help, then you've got a blockage somewhere.


Also, it helps if the drum brakes are adjusted properly prior to brake bleeding.

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The wheel cylinders have only one port for fluid to go in, and air to come out.  You have to build brake hydraulic pressure, enough to push the brake shoes against the brake drums, and when you open the bleeder, and with the brake pedal held down, wait long enough for the brake shoe springs to collapse the wheel cylinder, and push any air in the wheel cylinder back to the bleeder valve.


This is how I bench bleed the master cylinder.



After reattaching the truck's brake line to the master cylinder,  I needed to bleed some air out of the brake line.  The little cup and hose has a rubber end that seals over the brake bleeder fitting.  I crack open the bleeder fitting, and put the rubber end of the hose over the fitting.  Then I make sure the master cylinder reservoir is full, and step on the brake pedal.   The wheel cylinder fitting is only open enough that it takes about a second or two for the brake pedal to go all the way to the floor.  Then I hold the brake pedal down for about five to ten seconds.  521 wheel cylinders have only one port in them.  Any air in the wheel cylinder has to be expelled out to the bleeder by the brake shoe springs collapsing the shoes, and wheel cylinder piston.  you cannot use a pressure bleeder on a 521 because the brake system pressure is not relieved, and any air in the wheel cylinder just stays in there.



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Are these new drum brakes shoes you install or just the MC installed.

Maybe the brake shoes need to be adjusted as early Datsun are not self adajusting brakes.

Key is to know the before brake pedal and after pedal once the MC was installed.



start the truck and see if booster is working? Never had a boosted brake Datsun myself

Edited by banzai510(hainz)
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You don't say but it's assumed this is for a '74 620??


23 minutes ago, daretzcouple said:

Did you bleed the NLSV? Located under the passenger frame? 


Only the '76 and up trucks got the NLSV


The first self adjusting brake shoes were available half way through the '82 model year 720.

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Ok...thank so much guys.   I think I got it figured out.  The rears were pumping up so that’s what I assume was giving me a little bit of pedal, but come to find out the front adjusters were seized up and weren’t adjusted properly.  The piston on the cylinder was pumping, but as was said...it wasn’t pushing the shoes out enough to build pressure.  I removed the adjusters and got them cleaned and freed up so they now will move more freely.  I reinstalled  them and adjusted them so that the drum barely goes on then fine tuned them a little so that they barely scrub now. I bled them again and now have pedal pressure:) Took her for her maiden voyage and she stops pretty good. Problem is now as per the other thread...she died and wouldn’t restRt. Darn carb.  Thanks again and now gotta get it running reliably.





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