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Speedymaru

78 620. Replaced most parts, but the pedal goes to floor after sitting after bleeding

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Like the title says. I bled all 4 corners today, and the brakes felt good for an hour or so, and now the pedal goes nearly to the floor. I used the two person method, and also had a piece of vinyl tubing to help further prevent any bubbles. Yes, I ensured that all of the bleeders were tightened. Also, the pedal does return all the way to the top of travel on its own. 

I replaced the MC, and bench bled it. I replaced the rears with new drums, shoes, wheel cylinders, and hardware. I also had to replace the bleeders in the rear. In the front, I replaced the rotors and pads. 
 

I may be wrong about this, but my diagnosis is this: there is a leak somewhere in the system, and I’m thinking that it’s the front brake system that’s leaking because the pedal gets hard about 1-2 inches from the floor, which I believe is when it pushes mainly on the rear brakes. Apologies if that doesn’t make sense. Any help is appreciated. 

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Set the hand brake.  That should reduce pedal travel a lot.  If it does not, there is still sir in the system.  

521 brake cylinders have one port in the slave cylinders.  When bleeding the brakes, you have to allow the brake shoe springs to collapse the wheel cylinders the get air out of them.  If you close the bleeder with any pressure still in the system, air in the wheel cylinders will not get out.

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If there was a leak then you would see fluid dripping and the reservoir would need topping up.

 

 

Did you also bleed the NLSV the load sensing valve in the inside of the frame under the passenger seat?

1/ master front

2/ master rear

3/ NLSV front

4/ rear drums left first

5/ NLSV center

 

Are the rear shoes adjusted out so they just barely drag on the drums?

 

 

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My 521 master cylinder failed this way, fluid leaking out the front of it past the seals. Of course the reservoir would go down as it leaked...

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I’ll try the handbrake test today. Thanks

 

The truck is only parked on grass/mud so I may not notice a leak. 
 

I’m going to be honest, I didn’t even know the nlsv even existed. And yes, I adjusted the shoes so they drag a little against the drum. 
 

I haven’t noticed a drop in fluid in the MC, so that would be an indication that I do not have a leak. 
 

Thanks for everyone’s help. I’ll post back here in while with results. Thanks 

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Didn’t have a helper today so I couldn’t bleed them. I did notice a bit of brake fluid on the lines that connect to the MC. It wasn’t much though. I just replaced the MC. Something else I noticed was that when you turn the truck off, there’s a very slight “hiss” coming from the booster/MC area. 


I did the handbrake test, and it made no difference. 

 

One question, for Datzenmike: is that list that you posted the order that I’m supposed to bleed in? Just want to make sure. Thanks. 

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That's the order as set out in the '78 factory manual.

 

The hissing isn't right. The brake booster should be capable of storing vacuum. About all you can do is check the hose from the intake to the one way valve and from it to the booster reservoir is snug. It's normal to hear a slight hiss when using the brake as vacuum is exhausted through a felt diffuser into the cab, that's normal. If the leak was bad enough to cause the idle to falter or affect the brakes I would worry. For now just keep an eye on it.

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Thanks for the info. I should be able to get a helper today, then I’ll report back. Thanks

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Well here’s the update. I rebled everything, including the NLSV, and the brakes felt great. Then I notice drops of brake fluid dripping from the brand new MC I got from autozone. After doing some further research, it sounds like aftermarket MCs really are crap. I made a gasket for the place where it was leaking, and it now doesn’t leak. I attempted the bleed everything again, but couldn’t get the pedal to firm up. I’ll need to bleed more and hopefully that’ll fix it

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Well the gasket didn’t work out, so I went to autozone and apparently my MC is still under warranty so I ordered in a new one. Hopefully that should resolve my problems. Thanks for the help.

 

One more question about the NLSV: do you ever bleed the back one? I checked my manual and I couldn’t find any info about the NLSV at all. 

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On 4/17/2020 at 6:24 AM, datzenmike said:

 

Did you also bleed the NLSV the load sensing valve in the inside of the frame under the passenger seat?

1/ master front

2/ master rear

3/ NLSV front

4/ rear drums left first

5/ NLSV center

 

 

 

 

 

/3 says center, maybe it's the one you call rear.

 

What manual? because I only recognize the Nissan Factory Service Manual (FSM) which does cover it's operation. True, I have the '78 but it's assumed earlier ones were the same. The early 620 didn't have an NLSV until Aug '75 I think.This was also the month the long bed or long wheelbase truck came out.

 

The NLSV is positioned such that the front is slightly higher than the rear with a heavy weighted ball inside that rolls forward when brakes are applied reducing the flow of brake fluid to the rears. When weight is added to the box the rear settles lower effectively steepening the ramp the ball rolls up allowing more braking to the rear tires which are less likely to lock because of the added traction from the increased weight over then.

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Sorry for the delay. I got the new MC today. Haven’t bled anything yet. 
 

My manual is a Haynes, and it says it covers 73-79. I’m probably just incompetent and overlooked the portion that talked about the NLSV. 

 

Thanks for all the help! I’ll let you know when I get it bled and hopefully working. 

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Oh it mentions it alright but just that. If you want to do stuff on your 620 a manual that is 1/2" thick and cover 7 years of 620s is not going to do as good a job as the the Nissan Factory Service Manual (FSM) that is 1 1/4" thick and covers only one year of 620. The Haynes manuals just cherry pick info from the FSMs. My '78 FSM has the correct method of how and in what order to bleed the system but only for the '78.

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Makes sense. I’ll pick up an FSM soon. 
 

I bled the master and brakes today. They felt great a few minutes, then I turned the truck on and test drove it and they got spongy. I’m really stumped on this. I’m tempted to take it to a local Datsun shop. 

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On 4/22/2020 at 9:49 PM, Speedymaru said:

Makes sense. I’ll pick up an FSM soon. 
 

I bled the master and brakes today. They felt great a few minutes, then I turned the truck on and test drove it and they got spongy. I’m really stumped on this. I’m tempted to take it to a local Datsun shop. 

Did you ever get this sorted out?

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