620 front brakes help

[Hawaiian620]

Here's my problem. I changed all brake shoes and drums, then noticed the front brake cylinders(?) Leaking. I replaced those, bleed the system and everything seems tight. Until I start the truck and the brake pedal goes to the floor.

 

At first I couldn't see any other leaks. Had the wife sit in the truck and pump the brakes with it on. The pass side would move in and out. The drivers side seem stuck open and started leaking again.

 

I have no idea how to proceed. Any help would be appreciated.

 

Edit: Could the springs be too weak to close the piston? That's the only idea I have since I reused the old springs.

[Rick-rat]

How could you see the pass side move in and out with the brake drum on?

[Jester]

Bleeding brakes can take a while to fully get the air out. If you still have a leak, I would say it's time to pull that side apart again and see what's going on.Double check the rears as well just to make certain that the lines and bleeders are tight. 

[Hawaiian620]

I took the drums off the second time to see what was going on. They still got soft with the drums on. Noticed the leak and the piston not retracting on thr driver side ehen i took thr drum off. I checked the rears 3 or 4 times. I'll take apart the front again and see if there is debris or anything.

[datzenmike]

If leaking replace the leaking cylinders.

[Hawaiian620]

I did. The new one started leaking and stays open. I replaced booth front cylinders. The rears looked new and weren't leaking.

[datzenmike]

Perhaps the shoes are not adjusted? The cylinder is opening too far. Turn star adjuster from behind the backing plate until the wheel drags when spun by hand.

[Hawaiian620]

I'm starting to think that too. These are the first drum brakes I've done. I finally got around to googling some stuff and I think the pads are too loose. I'll tighten them up best I can after I reset the cylinder and hope that fixes things. Thanks for the help, again, Mike!

[DanielC]

You do not say what year 620, that info might help us help you.  I have 521 trucks, but the brakes on earlier 620 trucks are almost identical to 521 trucks, but the last years of 620 trucks are increasingly more different.

 

The basics.  521 brakes are not self adjusting.  Early 620 brakes may be, I do not know, I have not ever owned a 620, or worked on 620 brakes.   The brake backing plates must be clean, and a very tiny bit of grease on the points where the brake shoes contact the backing plates.  The shoe adjuster cylinder carrier must slide on the backing plate.  Not freely, but reasonably easily.  If you have brakes on both sides apart, the adjusters have left and right hand threads.  Left hand threads on the left.

 

To adjust the brakes, Block three wheels, front and back.  Jack the fourth wheel up, so the tire and wheel can spin freely.  Remove the dust cap off the adjuster.  If yours are missing, they might still be available at a Nissan dealer.  Turn the star wheel adjuster down.  This spreads the shoes.  Do this until you cannot rotate the tire and wheel forward, and back.  Go in the cab, and step on the brake pedal hard.  Now try to turn the jacked up tire and wheel again.   Adjust the brake shoes tighter again if you can spin the tire and wheel.  Once you found that point, back the adjuster off 12 clicks.  Ideally, you will not hear any contact between the brake shoes, and the drum.  Very light scraping is OK, drag is not.  Do the other three wheels in the same way, making sure the hand brake is off, and the transmission in neutral.

 

To bleed the brakes, you cannot use a power or pressure bleeder.  The wheel cylinders on these trucks have only one port into the wheel cylinder, and if there is air in the cylinder you have to depend on the brake return springs to collapse the wheel cylinder to push air bubbles out to the bleeder valve.  One way to do this is to have a helper push the brake pedal, and hold it down.  Then you open the bleeder, and let fluid out, and wait for the stream of old brake fluid to completely stop, before closing the bleeder.  Then your helper can let the brake pedal up.  You get three push the pedal, open the bleeder cycles, and then you must refill the master cylinder. 

 

Go for a test drive on a non busy street.  Lightly brake a few times, being ready for the truck to pull one way or the other.  Sometimes a light pull to one side can be helped by dragging the brakes a few hundred feet, left foot on brake, right foot on the gas.  Then see if the still pulls to one side.  If it still does, take the brakes apart again, and see what is different between each side.   If the truck does not pull, do a hard, and even a "panic" stop.  you want to know how the truck will act in that situation.

 

On my 521, stock brake system, the brake pedal sits well above the gas pedal with the tires locked from spinning.  

 

New brake shoes will have to be adjusted often. if you have not adjusted the brakes on your truck for a while, they will have to be adjusted often.  This is because the diameter of the brake shoes do not match the brake drum exactly, and if you keep the brakes adjusted, the shoes will wear to a perfect match.  When the shoes are out of adjustment, they wear on one end only, and if not fixed, will soon wear down to metal, and then you are trying to get new brake drums.

[Hawaiian620]

Daniel- Thanks for the info! I have a '77 620. I tried searching for most of what you wrote, but couldn't really find anything before I started on the brakes. I was sure I was missing something.

 

The  shoe adjuster cylinder carrier slides great on all four brakes, I bleed the system simular to what you said, but closed it before the fluid stopped. I'll try it your way and keep the cap open till the fluid stops. Great info on adjusting the dums and shoes. I will also re-adjust all four today. Like I said, I'm new to drums brakes and this is GREAT INFO! Thanks again!

[5t341tH]

I'm not sure, but does your year 620 have the NLVS(maybe thats how it is spelled)? load leveling brake bias adjuster, basically. I had to bleed mine there

[datzenmike]

'76 on had the NLSV I think.

[Hawaiian620]

Is that on the pass. side frame rail? Someone locally had told me about it, but I couldn't find it. I re adjusted all brake drums. Wheel cylinders are no longer leaking. All wheels feel good when tested off the ground. Pedal feels good when truck is off. I rebled everythinig.  As soon as I start the truck the pedal gets real soft. It feels a little better than before, but not by much. I'm guessing to need to find the NLSV and bleed it?

 

Did a search and found this thread you both posted in. Are there 4 different locations for the NLSV or are they all under the pass seat? Also, I've been bleeding from the rear to the front, then the master cylinder. Which I guess is wrong.

 

http://community.ratsun.net/topic/18748-1977-datsun-620-pu-brakes-grab/

 

More on NLSV from datzenMike

http://community.ratsun.net/topic/37658-proportioning-valves/

 

Loving the information on this site! I'll be rebleeding everything when I get off tomorrow. I'm confident that'll fix everything. I'll post up afterwards.

[datzenmike]

Test vacuum assist brakes....

 

Pump brakes several times to bleed off any residual vacuum in the booster.

Step on brake pedal and hold.

Start engine.

 

Here's what you should notice. The pedal drops slightly as soon as the engine starts and intake vacuum rises. This is normal. The vacuum booster assists you in depressing the brake master cylinder. The pedal and linkage by itself provides a mechanical advantage and an input on the pedal of 132 pounds produces an output fluid pressure of 850 PSI to the wheel cylinders. With the booster working, this increases to over 1,420 PSI. In other words you stop faster and easier with the same or less effort.

 

If the booster should fail you would revert back to the 850 PSI. The truck will take a significant amount more effort to stop, but the brakes still work.

 

Do not mistake a spongy pedal for vacuum assist in fact test the pedal firmness with the engine off. If spongy you have trapped air or the brake shoes need to be adjusted out more. The further the shoes have to travel to meet the drum surface the further you have to push the pedal down..

[G-Duax]

Also check for brake fluid in the vacuum pod on the power brakes.

(pull the master off the pod a little ways, it should be dry between the master between it and the pod)

When you said that the pedal goes down when the engine is started, the pedal goes to the floor, and you didn't say anything about a lot of leakage around the wheel cylinders, the first thing that come to mind is a leaking master, as I've had that happen.