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620 pickup, brakes aren't working properly


Kulper620

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I've got a 76 620 and when I press on the brakes the pedal goes strait to the floor, the brakes will start workin about half way down and it just slams on them but feels like there is no pressure on the pedal what so ever, bled the brakes a bunch of times and it stiffens up when the truck is off but as soon as I start the truck and press the pedal it goes strait back to the floor, any one have any ideas??? Thank you for any input

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Adam is trying to tell you how to make a first post. Note the introduction and a picture of your ride.... oh and politeness.

 

 

 

You have 4 wheel drum brakes. Lift each wheel and adjust the shoes closer to the drums. Turn adjuster until you feel the brake drag when spinning the wheel by hand. Pump the brake pedal to center the shoes, and adjust again (if needed)

 

Drum brakes need a lot of fluid to move them against the drum so if the shoes need adjusting the pedal will be traveling a long way to work them.

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mike wasn't saying you weren't polite ;)

 

but it is nice to see a pic of ur datsun, some plans, maybe a little history and an introduction as to who you are before you ask for help....

 

 

its just a preference around here, it normally obtains better results. 

 

 

 

 

 

ur brakes only have a few moving pieces(in theroy)

 

master cylider and wheel cylinders

 

 

have you bled your master yet? 

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I will try to get pics up soon, I've checked and bled everything I can so far, someone mentioned is while back that it could be the proportion valve but I'm not sure where it is located. Oh and my name is Jason, I've owned this 76 for 6 years and it's been out of commission for about three years cuz i let a buddy use it for a few days and he forgot to turn the choke off on my su carbs and threw a rod, finally got a motor just waiting for an oil pan to show up and it will be ready to fire up, only other issue I mite have is my drive shaft mite not fit due to the four speed I have is out of a 510, thanks for all the input

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I've got the same problem with the 620 I just bought.  I tried to adjust the brakes with the adjuster through the backing plate, but it just kept turning with no change in results (and yes I was spinning it the correct direction).  I had already planned to completely rebuild the brakes anyways, so I didn't go any further with it (I've got other issues to solve before I can drive it).  I think you might find you will have to pull the drums off and take a look to see what's going on.

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ok, heres a quick rundown on 620 brakes!

 

 

1st bleed your system

 

- master

- the NSLV (nissan load sensing valve) - found on the inner frame on the pass side (idf you have it) mine broke and i deleted it ;) 

rear

then front

 

always working your way from the farthest point towards the master 

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I've checked and bled everything I can so far, someone mentioned is while back that it could be the proportion valve but I'm not sure where it is located.

 

I don't have a proportioning valve on my '73.... It's been said on here that the master cylinder takes care of that internally.

 

This is for the '77 620, I'm not sure the '76 has the NSLV:

 

Scan0003.jpg

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then adjust your brakes, an old Nissan tech showed me this one

 

 

spin the adjuster for each wheel ALL THE WAY OUT till the wheel no longer rotates then adjust it back in till you can just hear the shoe scrapping on the drum

 

once its close hit the brakes a few times to center the wheel and do final adjustments

 

 

 

after all that you bleed it one more time and your brakes will lock up when mashed opn and put your teeth on the steering wheel if done right 

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I have a 521, and there are some similarities in the brakes.  The wheel cylinders only have one port for brake fluid to go in and out of.   On some cars, there are two ports into the wheel cylinder, a second one that goes to the bleeder, so air can escape easier when bleeding the brakes.  A 521 wheel cylinder has only one port, and I suspect a 620 is the same.

 

if you have pressure on the brake pedal, open the bleeder valve, you bleed the air out of the brake line, but unless you keep the bleeder open, and allow the brake shoe springs to retract, the air already in the wheel cylinder just stays there.  You have to let that wheel cylinder air back up out of the wheel cylinder, and out of the bleeder fitting.  I believe if you do this before adjusting the brakes, you have more wheel cylinder travel, and it is easier to expel the air bubbles in the wheel cylinder.

 

Try to bleed some more, giving enough time for the wheel cylinders to completely retract.

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brakes work on pascals law stating that in a sealed hydraulic circuit there should never be a pressure loss throughout the system. disconnect one of your lines coming off the MC and hook up a pressure gauge, read the PSI. like mike said, adjust your shoes with the starwheel adjuster until you feel a slight drag. check wheel cylinders for leaks. check brake hoses for leaks or kinks. if everything checks out ,do a good thorough bleed starting at the rear passanger and work your way to the MC until you see no air bubbles. if that doesn't do it check  your brake pedal height and adjust if necessary.

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I've never figured this out, so seems like a good time to ask:  Where does oics come from?

 

OICs came from a drunken mod that tried to type PICs and failed

 

 

 

you dont get many opportunities to poke fun at a mod - this is one of them

 

 

 

now its become second nature as ppl use it to fit in 

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Hi Jason, sorry I was not suggesting you were not polite. This is not always true of all first time posters though. We do have an odd sense of humor here.

 

The front/rear bias or proportioning is designed into the braking system by having larger or smaller rear wheel cylinders. When designing the 620 if the rear brakes were to lock up before the fronts, the engineers simply put smaller wheel cylinders on to reduce the braking allowing the fronts to do more work.

 

BTW... the pedal dropping when the motor is running is completely normal and just means your brake booster is working.

 

BTW... the '76 and on 620s had the Nissan Load sensing Valve NLSV. This mechanical device is... (somewhat) like a proportioning valve, but allows more braking to be applied to the rear wheels by sensing vehicle weight in the back. It needs to be bled in a proper order also.

 

 

Things that can cause a mushy pedal...could be a combination of two or more

 

The five flex lines, one for each wheel and one from body to differential, if old, will swell under brake pressure. It all adds up.

Shoes not adjusted properly

Air trapped in lines.

Air trapped in the master cylinder

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DanielC

 I believe if you do this before adjusting the brakes, you have more wheel cylinder travel, and it is easier to expel the air bubbles in the wheel cylinder.

 

 

 

I've never though about it like that before, that makes good sense.

I believe if you do this before adjusting the brakes, you have more wheel cylinder travel, and it is easier to expel the air bubbles in the wheel cylinder.

 

 

Jason, you said the truck has been sitting for 3 years....  If the brakes worked well before it sat, then more than likely it's not as much of a mechanical adjustment as it is a hydraulic issue....

 

***edited***

not sure where my quote went to, so I added it back in.

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Funny - looking at the page KlassicM posted ... they instruct us to work from the front to the rear.

 

 

Yeah, I've always been taught furthest rear to closest front, but the debate recently came up in another thread, so I checked the book..  Must have everything to do with how the NLSV works.

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Unfortunately this was an issue since I bought the truck, but the guy I bought it from had no clue about datsuns, I bought it for 750 because he couldn't get it to run correctly, fortunate for me I new the problem, he had the choke wired wrong paid him the cash and switched the wires and drove it out of his garage lol, he wasn't to happy

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I'm drawing a blank, should a power brake system be bled with the engine running?

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Power assist only makes it easier to push the pedal down. When driving it reduces fatigue. Try removing the vac line and see how friggin hard it is to work the brakes!!! For simply bleeding the brakes no assist is needed.

 

 

 

 

Yeah, I've always been taught furthest rear to closest front, but the debate recently came up in another thread, so I checked the book..  Must have everything to do with how the NLSV works.

 

This might have to do with drum fronts vs. front disc. Disc calipers don't move much, hardly at all where drums no matter how well adjusted are going to use more fluid.

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