* Brakes *

Dealing with all subsystems specific to the diesel powered Datsun-Nissan 720 pickup trucks.

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moose60
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Location: Seattle WA

* Brakes *

#1

Post by moose60 » 13 years ago

Since it has been raining like mad in Seattle this past week, I have found that the rear brakes on my 720 lock before the fronts. Sometimes quite easily, especially when traveling downhill. I realize that this is a pickup, but it seems like almost all vehicles are setup to lock the front brakes slightly before the rears.
1 I'm not pushing very hard on the brake pedal.
2.They do not seem to lock immediatly (unless I give the pedal a good push)
3. The front brakes seem to work normally. I can ride the skid, apply more pedal force and stop faster (by increasing the braking force of my non-skidding front wheels)
4. Rear tire pressures are 25psi.
Should I suspect the proportioning valve or simply the shoe adjustment for the rear drums?
The rain is keeping me from crawing under the truck.
Thanks
Byron
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82 Datsun 720 KC SD22

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asavage
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#2

Post by asavage » 13 years ago

Is the problem variable? Put another way, are the rear wheels more likely to lock up when you first start driving, or are they lock-prone all the time, about the same?
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

moose60
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#3

Post by moose60 » 13 years ago

Hmmm...
They may be somewhat more prone to sticking early in a drive, but the problem remains after repeated (5X) 15-20 MPH test stops.
The problem persists under my test conditions, and sometimes by accident while driving around. I am now keeping oodles of stopping distance, as it is disconcerting to come screeching up to a stoplight (or down a hill!).
Byron
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82 Datsun 720 KC SD22

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redmondjp
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Location: Redmond, WA

#4

Post by redmondjp » 13 years ago

Does your brake pedal feel firm and have normal travel (yes, these are subjective terms)? For the first 6 months that I had mine, my brakes were poor and I had excessive pedal travel--turns out that the master cylinder was shot. Replaced it, and wow! Super-firm pedal now, and requires virtually no pedal travel in order to stop.

This could be your problem--if your front braking system isn't working much if at all, the rear brakes are doing all of the work, and with the weight transfer towards the front during a stop, this then makes the rears lock up prematurely.

The shop that I go to checks f/r brake bias by applying brakes in their gravel parking lot while helper watches which wheels lock up. Pretty low-tech but it works!
1982 Datsun 720 King Cab, SD22, 86K miles (sold)
1981 Rabbit LS 4-door, 1.6D, 130K miles (sold)
1996 Passat TDI 4-door sedan, 197K miles

moose60
Posts: 168
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Location: Seattle WA

#5

Post by moose60 » 13 years ago

Red-
The pedal feels great. The brakes are responsive, action is firm and the effect can be felt fairly early in the pedal stroke. I've had a bad master cylinder, and bad slaves (brakes and clutches). I don't think that this is the problem.

Byron

I don't think that I have foot in mouth disease...
I hope I don't catch it from this post of mine.
Byron

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asavage
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#6

Post by asavage » 13 years ago

moose60 wrote:Hmmm...
They may be somewhat more prone to sticking early in a drive, but the problem remains after repeated (5X) 15-20 MPH test stops.
I was reaching for one or more leaking wheel cylinders, but they are always much worse after sitting overnight, and after a few applications they get better. Leaking wheel cylinders are extremely common in older brakes -- everybody's older brakes, not just Nissan.

I suppose this could be a faulty proportioning valve, but they fail so rarely.

Bad calipers -- siezed slides -- will sometimes brake evenly and poorly. This would be more common with the early (pre-'84?) than the later ones.

Nine times out of ten brake problems are fixed by going completely through the brakes from front to back. I know that's not what you want to hear, but the first thing I do on any car I am keeping is to replace all the hydraulics (except the lines, and sometimes I don't replace the MC). The wheel cylinders, if they aren't already leaking, have corrosion in the pistons and don't retract right, the springs are always shot because nobody's replaced them during any of the "brake jobs", the fluid is black and water-laden, the caliper slides are rusty and sticky, etc.

But that's not helping to diagnose your problem.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

redmondjp
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#7

Post by redmondjp » 13 years ago

You may be onto something with the proportional valve, Al. Maybe some debris or corrosion has blocked the front section of the hydraulic system.

Moose--have you cracked either of the front caliper bleeders open and had somebody press on the pedal to check for good flow? This is an easy test if you have a helper.
1982 Datsun 720 King Cab, SD22, 86K miles (sold)
1981 Rabbit LS 4-door, 1.6D, 130K miles (sold)
1996 Passat TDI 4-door sedan, 197K miles

moose60
Posts: 168
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Location: Seattle WA

#8

Post by moose60 » 13 years ago

Redmond-
I recently borrowed a vacuum bleeder and flushed the brake and clutch systems. The problem existed before and after the flushes. The fluid from the clutch lines was black and unsavory. The fluid from the brake lines was discolored, but not nearly to the extent of the clutch fluid.
I did not pump the pedal while I had the system open, because helpers were in short supply last weekend. :roll:

Everybody-
This may be unrelated, but here goes. Just before I bought the truck, the PO had a 'brake job' done. He said that it had gone metal on metal, and that the rotors and pads were replaced. I have always heard some squeaking from the L front wheel. I suspect a slightly dragging caliper. The squeak is present with cold brakes (no brake application, just driving slowly). If I stop once moderately, the squeak will go away for several minutes. If I stop very gently it will screech all the way to a standstill. A harder application will immediately stop the squeak. Also turning the wheel while driving slowly can alleviate the squeak, for the duration of a swerve.
Related?
Byron
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redmondjp
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#9

Post by redmondjp » 13 years ago

That squeak you're talking about is probably just metal-on-metal vibration, between the caliper piston and back of the pad, between edge of the metal pad backing plate and the caliper, or between caliper and its supporting bracket. Usually solved by applying a very small does of high-temp grease or anti-sieze compound to the metal surfaces that are in contact with each other. Often times, the brake shops seem to skip this important step of lubricating these contact areas.

OK, next idea--sticking front brake circuit hold-off valve (not 100% sure if this truck uses one)--see this website for a picture and description (scroll down a bit).

Most of these valves have a small pin which sticks out that moves outward slightly (maybe 1/16" tops) when the brakes are applied--if you can find this valve (often on the front crossmember of older domestic cars), make sure that the pin moves freely and isn't stuck. This valve allows pressure to build up slightly in the rear (typ. drum) system first before applying the disc brakes, as disc brakes take less pressure to start working (because of the larger area of the piston vs. the tiny pistons in the wheel cylinders).

A faulty hold-off valve will definitely throw the f/r braking bias off.
1982 Datsun 720 King Cab, SD22, 86K miles (sold)
1981 Rabbit LS 4-door, 1.6D, 130K miles (sold)
1996 Passat TDI 4-door sedan, 197K miles

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asavage
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#10

Post by asavage » 13 years ago

redmondjp wrote:That squeak you're talking about is probably just metal-on-metal vibration, between the caliper piston and back of the pad, between edge of the metal pad backing plate and the caliper, or between caliper and its supporting bracket.
Or cheap, hard pad material. Swap out the pads for NAPA's United (not True-Stop or the intermediate brand, but the top-of-the-line pad: United, line code "UP").
This valve allows pressure to build up slightly in the rear (typ. drum) system first before applying the disc brakes, as disc brakes take less pressure to start working (because of the larger area of the piston vs. the tiny pistons in the wheel cylinders).
Um. No.

First, the larger piston in a disc brake caliper requires less volume to actuate but more pressure than a drum's wheel cylinder! In hydraulics, reducing the master cylinder size reduces pedal effort and increases pedal throw, and the reciprocal is true of the slave cylinders.

Second, drum brakes require substantially less hydraulic pressure to provide as much stopping force as equivalent disc brakes. That's the reason for the proportioning valve: to direct less pressure to the rear drums than to the front discs. You have to squeeze pads to the rotor much harder than you think, to get reasonable stoppage. Disc brakes have two main advantages over drums: they can dissipate more heat, faster, and are faster/cheaper to service. They have two main disadvantages too: rotors warp a lot more often than do drums, and asymetrical calipers tend to seize in the slide area. The latter is not an issue with caliper designs that have pistons on both sides of the rotor -- all 720s (AFAIK) have single-piston calipers that slide in the spindle.

Drum brake shoes have a LOT more swept area than any disc brake (esp. on the old Nissan vehicles: have you seen how tiny the front pads are?). And the leading shoe (the one installed toward the front of the vehicle, also the one with slightly less actual friction material on it) tends to self-energize: the leading edge is "grabbed" by the drum and pulls it even tighter to the drum.

Next: residual hydraulic pressure. Wheel cylinders require a few PSI to maintain their seal. If a small amount of pressure is not maintained, they leak. Calipers, OTOH, don't require any residual pressure to seal, and will drag if there is any. So the master cylinder on a disc/drum system has a pressure check valve that feeds the line to the rears, but no check valve to the fronts. That valve maintains residual pressure to keep the rears from leaking.

On the hold-off valve: ECI's site says, "These valves are used for the front disc brakes in a disc/drum braking system to provide a "hold-off" feature so as to allow the rear drum brakes to actuate first. This function is essential for correct system operation. "

I think that's almost complete BS. While Ford and GM did use a hold-off valve on some 70's vehicles -- F250 and Chevelle come to mind -- the purpose was not as ECI descibes, nor do I agree that a hold-off valve is "essential", as most cars don't have one. The rear drums to not have to actuate first, but generally will anyway, unless the proportioning valve is very well set up.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

redmondjp
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Location: Redmond, WA

#11

Post by redmondjp » 13 years ago

Well I guess we'll have to agree to disagree on the purpose or need for this hold-off valve , Al. I have owned close to 20 american cars and trucks (Ford, GMs of all flavors, Plymouth) with front disc/rear drum brakes, and virtually every one of them had this hold-off valve in the front disc brake system, right on the front crossmember. Some time in the mid-1970s, GM incorporated this valve right into the front of the proportioning valve, where you can see the rubber boot and small pin sticking out. And if you have a helper actuate the brake pedal, you can watch this pin extend from the hold-off valve when pressure is applied to it.

Another explanation that I've heard for the purpose of this valve is to allow the pressure in the rear system to build up and push the shoes out toward the drum, such that the front and rear brakes apply at the same time. Unless one is diligent about manually keeping the shoes tightly adjusted in a drum brake system (I used to do it about twice per year, when rotating tires or installing/removing winter tires), there is always some clearance between the shoes and the drum which needs to be taken up before the brakes apply. On discs, very little piston travel is needed to apply the brakes, which are lightly dragging all of the time. The self-adjusters used in drum brakes just don't work that well--if you want the least amount of brake pedal and parking brake pedal/handle travel, you usually have to manually adjust the shoes.

At any rate I was just trying to help figure out this strange brake problem and didn't mean to steer this thread into a brake system discussion, especially if it doesn't help to solve the problem at hand.
1982 Datsun 720 King Cab, SD22, 86K miles (sold)
1981 Rabbit LS 4-door, 1.6D, 130K miles (sold)
1996 Passat TDI 4-door sedan, 197K miles

moose60
Posts: 168
Joined: 14 years ago
Location: Seattle WA

#12

Post by moose60 » 13 years ago

Ughh.
Brakes.

Both rear drums had/have the shoes dragging. I even locked 'em today in the dry on new concrete, which is what prompted me to crawl under.
The adjuster is frozen on one of the sides. I'm guessing that I've gotta pull the drum to free the adjuster. Sound right?
My old special brake adjuster lever (with handy bends) seems to have disappeared since I got rid of all the VWs several years ago.

Tonight a memory crept upon me unbidden:
A few weeks ago, I got off of the fishing boat that I work on around 4:00AM after we had several hours worth of hydraulic trouble. Since we had no hydraulics, and thus no refrigeration, I got several loads of ice for the fish. We were both zipping back and forth, toting ice. By the end we got a little rowdy. Somebody burned out, I did an e-brake slide(I realize that this isn't the best auto for this).
Could this cause my troubles with the rear brakes locking. In my (somewhat) younger days I have use the e-brake to great success in many vehicles. Is the 720 somehow different.
Could this cause have caused my lock-up problem?

Bedtime.
Byron

82 Datsun 720 KC SD22

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asavage
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#13

Post by asavage » 13 years ago

I used to commute across the Clinton-Mukilteo ferry to go to school in Everett. I kept various beaters on the Mukilteo side, and would walk across. You couldn't keep a decent car there because of vandalism.

One of the cars was a '66 VW Beetle, a very rusty car. So rusty was the A pillar that there was no place for the door's lower hinge to mount.

Anyway.

Inevitably, I'd be running late coming home to catch the ferry back to the island, and I had to park nose pointed in the reverse direction to the way I'd pull up. In wet weather, I'd shortcut turning the Bug around: while still travelling at 20 MPH, I'd crank the wheel slightly left while yanking on the park brake lever. Rear wheels lock, rear would do a perfect 180, pull forward into park spot.

After several of those "turns", the brakes began acting odd. Pulled rear drums (a LOT harder on old Bugs than on our 720s) and found bent brake shoes on both sides.

Summary: if you have brake problems, and if you're going to keep the truck, go through the brakes . You're likely to find several things that need attention, not just the one frozen adjuster.

Don't forget to grease the shoes. If you don't know what I'm talking about and think I'm nuts, read up on proper brake service. I use high-temp caliper grease for shoes. Adjusters, I take apart, clean with cleaner, then buff on wire brush wheel, reassemble using anti-seize. I'm sure others have their own tips.

But don't forget to grease the shoes.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.

EvergreenSD
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Location: Eugene, OR

#14

Post by EvergreenSD » 13 years ago

My rear brakes were an absolute nightmare when I got the truck. The stupid thing is that I didn't realize how bad it was because the fronts were still working. If I needed to stop it would stop, but the fronts would lock up easily. Upon inspection, the rear brake line was CUT, the parking brake cable was cut and one adjuster was stuck; 100% stuck. I grabbed an adjuster off a gasser 720 and the JY gave it to me for free. You're going to have to take a look back there.
'82 SD22 720 Kingcab with flatbed

moose60
Posts: 168
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Location: Seattle WA

#15

Post by moose60 » 13 years ago

*Splash*

Ok, I'm diving in. I've just gotta find a spare bolt of the correct size to drive the drums off.

Al- I learned the exact same maneuver in my 69 bug. I mostly did it when the ground was wet, or I was on gravel. That thing was a little go-cart.
Byron

82 Datsun 720 KC SD22

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