Rear Brakes Locking up 85 720

[IZRL]

While driving at 10 mph, if I press the brakes with even slightly more than regular pressure. My rear brakes lock up. I've bled the brakes and adjusted the rear shoes. I don't seem to be losing any brake fluid but not 100% sure I don't have a leak. I'm about to order drums, shoes/hardware, front brake hoses, and wheel cylinders. Wondering if there's anything else that might be causing the problem?

[datzenmike]

What led up to this?

 

Were they working just fine one day and doing this the next???

 

 

Are the shoes wet or contaminated?

 

 

 

 

[Thomas Perkins]

Mine did that long time ago.I would remove the little black plug on the inside backing and use a flat head screw driver and turn the the star adjuster and it would fix it till I drove it some more.I could not figure out what was wrong.

I took it to Jack,my go to Nissan specialist.The problem was the driver side emergency brake cable.

It was rusted out on the end coming into the hub.You can't see it by looking at it with the  hub off.That was causing it to lock up.I did replace the shoes, springs.Jack bought the last one Nissan had back then,I think it was 75 bucks.I also think that they have different lengths according to your truck size, regular cab, king cab.I also put new hubs on their.Use 2 long bolts to remove hub.Screw into 2 threaded holes evenly and hub will come off.Remove the cable and check out the end.

 

Edited December 29, 2023 by Thomas Perkins

[IZRL]

3 hours ago, datzenmike said:

What led up to this?

 

Were they working just fine one day and doing this the next???

 

 

Are the shoes wet or contaminated?

 

I just started test driving the truck. I've taken it out maybe 3 times. It was parked for about 5-6 years. I cleaned all brake components with brake cleaner so it's clean as a whistle. Neither shoes nor cylinders were wet when i removed the drums.

2 hours ago, Thomas Perkins said:

Mine did that long time ago.I would remove the little black plug on the inside backing and use a flat head screw driver and turn the the star adjuster and it would fix it till I drove it some more.I could not figure out what was wrong.

I took it to Jack,my go to Nissan specialist.The problem was the driver side emergency brake cable.

It was rusted out on the end coming into the hub.You can't see it by looking at it with the  hub off.That was causing it to lock up.I did replace the shoes, springs.Jack bought the last one Nissan had back then,I think it was 75 bucks.I also think that they have different lengths according to your truck size, regular cab, king cab.I also put new hubs on their.Use 2 long bolts to remove hub.Screw into 2 threaded holes evenly and hub will come off.Remove the cable and check out the end.

Im going to double check and adjust the parking brake cables see if that helps. I didn't even think about having to replace parking brake cables yet, I have the 4 door/crewcab 720 and I'm guessing they're longer than the king cab cables? I'm going to have to do some research about that. thanks

Edited December 30, 2023 by IZRL

[IZRL]

Nothing else you guys can think of then? Just making sure before I put in the order. Thanks

[IZRL]

Just found out there might be an NLSV on this truck. I followed the lines all the way from the front to the rear and I didn't see it. I also can't find a replacement for it. I'm going to put in the order for the parts I mentioned. I'll install and adjust everything, then ill report back to let you guys know if that fixed the problem.

[datzenmike]

1/  When the brakes lock up as you describe, raise each rear wheel and see if they will spin easily by hand. 

 

2/ When driving under the locking brake conditions do the rear RIM(s) become hot to the touch?

 

 

3/ check the front reservoir brake fluid level in the master cylinder. If dry and the front disc brakes are not doing any work when stopping is all done by the rears.

[IZRL]

26 minutes ago, datzenmike said:

1/  When the brakes lock up as you describe, raise each rear wheel and see if they will spin easily by hand. 

 

2/ When driving under the locking brake conditions do the rear RIM(s) become hot to the touch?

 

 

3/ check the front reservoir brake fluid level in the master cylinder. If dry and the front disc brakes are not doing any work when stopping is all done by the rears.

The brakes don't stay locked up. I can immediately start driving normally after the stop. It's more like if you stomp on the brakes of a vehicle without abs. Only I'm not stomping on the brakes. Anything more then easing into a stop locks up the rears and it goes into a skid. The master cylinder level is right below max and it hasn't dropped at all.

[datzenmike]

The engine would easily overpower what I'm looking for. The rear shoes may (or maybe not) be dragging. Not enough to remain locked up just applying some braking. When you add your braking the wheels lock. Just humor me and raise and try to spin the rear wheels.

 

 

take for a drive... do the rims get hot?

[IZRL]

46 minutes ago, datzenmike said:

The engine would easily overpower what I'm looking for. The rear shoes may (or maybe not) be dragging. Not enough to remain locked up just applying some braking. When you add your braking the wheels lock. Just humor me and raise and try to spin the rear wheels.

 

 

take for a drive... do the rims get hot?

I'll get on this tomorrow morning and report back thanks. 

[IZRL]

Locked up the wheels purposely multiple times and they release immediately afterwards, no resistance. I also took it out for a ride and the wheels are cool to the touch.

[Thomas Perkins]

On 12/29/2023 at 7:49 PM, IZRL said:

 

I just started test driving the truck. I've taken it out maybe 3 times. It was parked for about 5-6 years. I cleaned all brake components with brake cleaner so it's clean as a whistle. Neither shoes nor cylinders were wet when i removed the drums.

Im going to double check and adjust the parking brake cables see if that helps. I didn't even think about having to replace parking brake cables yet, I have the 4 door/crewcab 720 and I'm guessing they're longer than the king cab cables? I'm going to have to do some research about that. 

 

[Thomas Perkins]

Where did you get that 4 door 720 from?

Edited January 1 by Thomas Perkins

[IZRL]

20 hours ago, Thomas Perkins said:

Where did you get that 4 door 720 from?

A friends grandfather imported it from Saudi Arabia while he was with the military. My freind inherited it when he passed. And I purchased it from him a little less than a year ago.

 

Measured the wheel base and its 9'3". Same as all 4wd 720s.

[Thomas Perkins]

Awesome.

[IZRL]

Well I'll be damned! I don't want to jinks it🤞 but the rear brakes aren't locking up anymore! I have no idea if I had anything to do with it. Or if it just needed to be driven enough to get the cobwebs out? What might've fixed it and that's a big maybe? I was thinking maybe the front pistons weren't working properly from sitting for so long (I'm no brake expert so this theory could be total bologna). In any case I removed the calipers and pressed the pistons like if I was going to fit new brake pads and then just cleaned/greased everything up and put it back together. Then I took it out for a test run. The first 4 hard stops it still locked up but I noticed I was able to put slightly harder pressure on the pedal before it locked up. After that, it was stopping like a champ. Who knows if this even fixed it, or if I just needed to drive it more to get the juices flowing? But there you go, worth a try if anybody else experiences this problem.

[Stoffregen Motorsports]

I have a similar issue with my 77 Cherokee rear brakes locking up when it's wet or even moist out. When I rebuilt the rear axle/brakes, everything went together like it should, and I haven't had a chance to see what the heck the problem is, but like I said, if it's wet out, the rear brakes will lock for the first couple of braking attempts, even with very gentle brake pedal pressure.. I suspect it has something to do with the proportioning valve. If I stab the brakes really hard, the problem lessens. When I start driving, the problem goes away.

 

I did, however, have it happen once last winter while driving in the snow, after I had been driving for a while. So something to do with cold temps?

 

 

[IZRL]

43 minutes ago, Stoffregen Motorsports said:

 

I did, however, have it happen once last winter while driving in the snow, after I had been driving for a while. So something to do with cold temps?

Could be. I had already ordered all new brake hoses, wheel cylinders, drums,pads and hardware. So maybe after replacing all that and adjusting the parking brake, this problem won't come back? I followed all the lines and I didnt find an NLSV. Although after quite a bit of research I still don't know if this truck even came with one of those? I'll keep my fingers crossed that was the end of that.

[datzenmike]

All 720 had the NLSV. It's mounted with the front tipped up slightly with a heavy steel ball and spring. Under braking, the ball rolls up hill and restricts brake fluid to the rear brakes. As weight is added the rear squats down increasing the ramp angle. The ball has a harder time rolling up the ramp, and more fluid pressure is applied to the rears. The increase in carried weight increases the tire traction so less chance of lock up and more brake can safely be applied to the rears.

 

The NLSV is located on the inside of the frame rail under the passenger side.

 

If it was removed, I can't see why, then full braking pressure would be applied to the rears and lock up is almost guaranteed on moderate and hard stopping. If it's gone a Wilwood proportioning valve in line could be adjusted so that when stopping hard the rears just lock up when the fronts do.

[banzai510(hainz)]

drive backward fast then slam the brakes. maybe it neede to recenter itself

[IZRL]

4 hours ago, banzai510(hainz) said:

drive backward fast then slam the brakes. maybe it neede to recenter itself

As crazy as this sounds it probably wouldn't hurt to do this at a reasonable speed. Hahaha

4 hours ago, datzenmike said:

 

The NLSV is located on the inside of the frame rail under the passenger side.

Is there a photo of where exactly this illusive little bugger sits and what it looks like? My brake line looks to be routed in its stock position. The line doesn't go into the frame and the line doesn't have a splitter of any kind that would show someone removed it. I spoke to the friend who sold me the truck and he said it wasn't locking for the short period he drove it. So for sure this happened from sitting for so long. 

[datzenmike]

Not in the frame but on the inward side of it. Bolted to frame tilted up slightly at the front, one line in one out and a bleeder on the top. Roundish shape.

 

Of the three causes of extreme rear wheel lock up in the factory service manual it looks like number three...

 

1/ improper tire pressures

2/ excessively worn tires

3/ damaged NLSV

 

Possibly it was just stuck from sitting and is now just fine.

 

[Stoffregen Motorsports]

The factory proportioning valve (LSV) can get sludged up inside, making it malfunction. I just took one apart the other day and it was very nasty inside. It is possible to clean it without disassembling it I suppose. Brake cleaner and some compressed air might do the trick.

 

I found this image on another site. Is this the valve and location Mike?

 

[Thomas Perkins]

Never had a problem with mine.Never ever bled it.I say it is your emergency brake cable locking up.Take it off and check out the end piece.Had my 720 for 29 years with 391,000 miles on it.Only people that had this problem will know. this.This was about 15 years ago when this happened.It will get worse in time.

 

[Thomas Perkins]

I just went out to take of a picture.Never touched it before.It may be your problem.Jack put the new driver side Emergency brake cable on back then.He replaced the front rotors,axle boots,front bearings new master cylinder new brakes hose.Back breaks were good.New calibers.That was about 4 years ago.He might of bleed that valve .I changed brakes many times and never did it.

Edited January 6 by Thomas Perkins