620 Rear End Diagnosis

[Randalla]

I have a 77 620 KC that I'm trying to bring back from the dead. It was a work truck for 40 years, and as such was likely abused. I've replaced most of the wear items. Truck is running strong. I've only driven it in my neighborhood so far. It drives good for a short distance then I hear a loud sound in the rear end, and the truck acts like the emergency brake has been applied. If I jack the rear end up after this happens both rear wheels are locked in position in both directions. If I gently move the wheels back and forth a little, they release, but then will freeze back up with further turning. My initial thought was that somehow the adjusters were tightening up when driving, but I've not had that happen before on other Datsuns I've owned. Also thought maybe the differential might be damaged and locking up. Any thoughts where to start looking would be appreciated. Or anyone who may have had a similar experience. Thanks guys.     

[datzenmike]

Pull the top bung out and see if it even has oil in it. You should be able to touch the oil with little finger. If you haven't changed the differential drain it now. This will tell you the most if there are chunks of metal in the oil and have a good look at the bung it has a magnet on the end.

 

620 brakes can't self adjust. The '81 720 truck was the first year that self adjusting read drum brakes were used.

[thisismatt]

Your brake pedal may not have enough free play. There should be some free play between the pedal and the master cylinder push rod to guarantee the cylinder returns fully, otherwise residual pressure can increase and lock up the rear brakes. 

[Randalla]

I previously drained and refilled the diff with fresh lube within the last 30 days. There were no chunks or metal particles on the magnet. I will check for free play at the pedal tomorrow and report back. Thanks for your thoughts.  

[datzenmike]

Was the transmission in neutral???? and still no movement???? If transmission was in gear and one side was jammed the other side would not turn either. It's the only combination that fits the facts.

 

If in neutral, one side could be jammed (brakes were probably replaced but something came loose and is jamming things on one side or the other) but the other wheel would spin the driveshaft.

 

If pinion and crown gear jammed the tires would spin in opposite directions.

 

 

 

 

[ratpatrol66]

Speaking of rear end diagnosis. I need to see my doctor and have my prostate checked. Thanks for the reminder. 

[datzenmike]

 

My last one, I was looking forward to, as my old male doctor had retired and a younger blue eyed blond woman from South Africa (with an incredible sexy German accent) had take over his practice. I was almost excited at the prospect of this woman checking out my most Holy of Holies, my inner sanctum, my....... but I digress.  

 

So I am ushered into her office and this... hag, walks in and introduces herself while putting on gloves. Wait! What??? My doctor is on holiday and 'Helga', the she wolf of the SS, is filling in for her absence and is going to examine me????? Ohhhh man. At least she wasn't brutal, but made up for that by being clinical and very thorough. In the after glow, she spoiled everything by saying there was nothing to be embarrassed about but I think it was her that was embarrassed. Maybe it was her first rodeo other than a cadaver but it wasn't mine..... and.... the moment passed. 

[Randalla]

😆 Didn't realize the double entendre till now. Thanks for the laugh this morning Mike. Wish I was also laughing about my 620. Still trying to diagnose. Going to remove the wheels and drums today, jack the rear end, start the car, drop it in gear, to see if I can isolate a brake issue from something more sinister.

[datzenmike]

On 10/7/2022 at 6:09 AM, datzenmike said:

Was the transmission in neutral???? and still no movement???? If transmission was in gear and one side was jammed the other side would not turn either. It's the only combination that fits the facts.

 

If in neutral, one side could be jammed (brakes were probably replaced but something came loose and is jamming things on one side or the other) but the other wheel would spin the driveshaft.

 

If pinion and crown gear jammed the tires would spin in opposite directions.

 

 

 

 

 

I don't see it being the brakes, certainly not the hydraulics. Could be something replaced fell out and jammed between shoe and drum. Cannot see it happening on both sides. Save yourself some time. Place in neutral and raise both wheels. Ones side is jammed.

[Randalla]

Mike, had to satisfy myself that the lock up was not the brakes. After running the truck with the drums and wheels off I've yet to have the rear end lock up like it was doing previously. I'm going to keep playing with it to see if it happens again, but in the mean time, I'm leaning toward my own stupidity, possibly having assembled something in the rear brakes incorrectly. Please take a look at  the pics below and let me know if you spot anything obvious (shoes reversed or upside down for example).  The first three pics are one side of the truck, and the last three the other. Thanks 

[datzenmike]

The shoes can only go on in one position because of the horizontal 'link extension' just below the wheel cylinder. The lining positions are identical on all 4 shoes. Looks good to me.

 

Did the drums pop right off? I would think that if it's the brakes they would not let go.

 

Again if one side jammed it would not affect the other side.

[Randalla]

Arghhhhh! Was actually hoping I did something stupid that would be easily remedied. The drums did come off fairly easily. May try to back off the adjusters a little. This one really has me baffled as well as my local buds. 

[datzenmike]

Put together and keep going till it does it again I guess.

[Randalla]

OK, now have a few more data points that raise further questions. Backed off the rear adjusters completely and ran the truck with the wheels off but drums on (on jack stands). It eventually locked on the passengers side, tight enough that it stalled the engine and was extremely difficult to get that drum back off. When I took the drum off I noticed that one side of the brake cylinder was way out vs. the other. I checked the driver's side and it looked similar but had not yet locked. So, clearly the brake cylinder is the problem, as it is locking the shoes against the drum. Both brake cylinders are brand new, so that leads me to wonder if they can be put on upside down, or if there are right and left sides??? In the pictures below you can see what I'm dealing with. Any thoughts?  

Edited October 11, 2022 by Randalla

[datzenmike]

Never seen the likes of that. I love a mystery

 

For the wheel cylinders to extend like that there must be hydraulic pressure. Hydraulic pressure comes from pushing down on the brake pedal.

 

I assume you replaced the master cylinder???

[Randalla]

Yes, master cylinder is new as well. Does a 620 have a proportioning valve like a Z car? I haven't seen one, at least not in the engine bay. Could the one way booster valve possibly have anything to do with this (over boosting the rear brakes?). Could a bad booster cause this? I'm a fairly good mechanic, and have owned more than 20 Datsuns, but I've never seen this before.

Edited October 11, 2022 by Randalla

[Randalla]

I should point out this was someone's work truck for 40+ years, and I've found a number of mods on it that I removed. It did have trailer brakes at one time. It's certainly possible the previous owner did something that would cause these issues. 

[datzenmike]

 I don't believe it has a proportioning valve. Any front/rear bias is designed into the master cylinder.

 

Willing to try an experiment? Pull the hose off the brake booster and plug the engine end so there is no vacuum leak. Be aware that the braking is now severely compromised and a lot of extra effort will be needed to apply the brakes so don't do down the highway. The brake will still work but you have no assist and the pedal very stiff.

 

The booster has a large diaphragm with vacuum on both sides so it in a neutral state. When you touch the brakes it not only works the master cylinder but also introduces a vacuum leak on one side of the diaphragm. Ambient air pressure begins to push on the diaphragm adding to your foot pressure to apply the brakes. Now if there was a small vacuum leak the brakes might apply even if you are not stepping on the pedal. The engine would have to be running of course.

 

Get the brakes loose. Roll the truck back and forth by hand to be sure. Start the engine and let idle for a while and the brakes should apply without you doing anything. Keep rolling it back and forth. If the brakes lock up then the booster is the cause.

[Randalla]

Willing to try anything at this point. At least I now know what's causing the rear end to lock up. Just need to figure out what to do about it. Appreciate the suggestion and will try that tomorrow morning. Thanks Mike.

[Ranman72]

the same thing mike said but also there should be a one way vacuum valve in the brake booster to motor line 

the absence of this or reverse setting in the line or even a faulty one could cause this possibly  

if this is exactly what mike was saying my apologies  

[datzenmike]

The one way valve only hold vacuum in the booster when the engine is off. This allows you to use the brakes one or two pumps should the engine fail/stall while driving or when you step on the brake to hold the truck when starting. After that the vacuum is exhausted. If you are in this situation use the one or two chances to stop normally wisely. Don't pump the brakes.

 

I had the vacuum hose off on my 620 and I guess a bug crawled in there for the winter. There was no vacuum getting through to the booster and I drove out the driveway normally and stepped on the brake and very little happened. To get stopped I had to put both feet on the brake. Very scary. A replacement valve fixed it right up.

[Ranman72]

2 hours ago, datzenmike said:

The one way valve only hold vacuum in the booster when the engine is off. This allows you to use the brakes one or two pumps should the engine fail/stall while driving or when you step on the brake to hold the truck when starting. After that the vacuum is exhausted. If you are in this situation use the one or two chances to stop normally wisely. Don't pump the brakes.

 

I had the vacuum hose off on my 620 and I guess a bug crawled in there for the winter. There was no vacuum getting through to the booster and I drove out the driveway normally and stepped on the brake and very little happened. To get stopped I had to put both feet on the brake. Very scary. A replacement valve fixed it right up.

i had a similar thing happen in af350 crew cab long bed 4wd no vacuum but they used the power steering pump to make the hydraulic assist brakes

i blew a power steering line and the steering was ridiculously hard and the brakes took both feet while pulling on the wheel 

not a fun experience but we survived  

[iceman510]

Why are the boots not in the piston grooves the same on the opposite ends?

 

Image below is a new cylinder.

 

 http://cdn.shopify.com/s/files/1/0290/2648/4308/products/375-67022.jpg?v=1662139675

 

 

On 10/8/2022 at 6:40 PM, Randalla said:

Mike, had to satisfy myself that the lock up was not the brakes. After running the truck with the drums and wheels off I've yet to have the rear end lock up like it was doing previously. I'm going to keep playing with it to see if it happens again, but in the mean time, I'm leaning toward my own stupidity, possibly having assembled something in the rear brakes incorrectly. Please take a look at  the pics below and let me know if you spot anything obvious (shoes reversed or upside down for example).  The first three pics are one side of the truck, and the last three the other. Thanks 

 

 

 

 

 

 

Edited October 11, 2022 by iceman510

[datzenmike]

 Pulling the hose off the booster should release the brakes.

 

 

[Randalla]

Another data point. I was able to free the brakes up, coax the brake cylinder pistons back into their bore, re-install the drums, and disconnect the hose from the booster to the intake manifold (capped the vacuum pickup tube). I ran the truck through the gears (forward and reverse) while applying the brakes periodically. The brakes have yet to lock. I'll continue this exercise to assure myself the booster may be the source of the problem. I also verified the one way valve was installed in the correct position.