720 won't go forward

[Mrs. Cold]

Hi everybody,

 

My name is Christian, I am posting under the name "Mrs. Cold" because that's what my SD-22 does.

 

I've got an 81 720 with an SD22 that I recently had rebuilt.  Got new bearings, pistons, liners.  Had the head welded in a heat tank and then totally rebuilt.  I got everything put back together with help from Wayno on the datsun diesel site but there's very little traffic over there and the engine is running like a top now, so what I need is truck help more than motor help and I am hoping there will be more traffice here.   This is the truck:

 

Hmmm,

 

I tried to post a pic but I don't have permission, here's a link to it

 

 

https://lh5.googleusercontent.com/-y_n1SydzprA/TlHFXNTXrmI/AAAAAAAAAWY/oMuV1UehWM0/w800-h533-no/IMG_3368%2B%28Medium%29.JPG

 

 

[Mrs. Cold]

Here's a new one for me.

 

I am slowly troubleshooting everything my 720 needs to get back on the road.  I just finished putting in a fresh engine and started working on the brakes.  Yesterday I was able to drive the truck forward in 1-3 gears and reverse.  The brakes were really bad so I started working on them.  

 

I bled all four brakes, no help.  Truck still goes both F and R.

 

Today I started checking the brake booster.  Bad valve between the vacuum source and the booster.  No vacuum is passing.  So I bypass the valve and hook the vacuum directly to the booster.  And then I backed up about ten feet.  The brakes work fine in this config.  Then I try to go forward and the truck stalls.  I start it and go forward again and this time as I let off the clutch, it begins to stall so I push the clutch back in.  Each time I try to go forward, the truck acts like the E-brake is on.  As I let off the clutch, RPMs fall until it stalls.  If I rev up, I can feel the car straining, as if I am trying to drive over a cinder block or push something heavy, and then the revs fall until the motor stalls.  

 

But not in reverse.  In reverse the truck acts just like normal.

 

Thinking that futzing with the brakes caused this I unplugged the vacuum from the booster.  No change.

Thinking that there was a wheel somehow stuck that could spin only in one direction I jacked the truck up and spun them by hand.  All wheels spin freely both ways.

 

I am thoroughly flummoxed.

[datzenmike]

Hi Christian, welcome to Ratsun. I combined your two posts into one so there is some background info here.  Wayno also lives here, but you probably know this.

 

As to the forward gears not working, my guess is that it's still in reverse.  Try placing it in neutral and let out the clutch, ... bet it backs up.

[MicroMachinery]

Has 2 posts.. post count is still 1.

[wayno]

Welcome :)

Are you the gal I met at the pick and pull in Sherwood, where you were having issues moving forward because the clutch was slipping?

Hi everybody,

 

My name is Christian, I am posting under the name "Mrs. Cold" because that's what my SD-22 does.

 

I've got an 81 720 with an SD22 that I recently had rebuilt.  Got new bearings, pistons, liners.  Had the head welded in a heat tank and then totally rebuilt.  I got everything put back together with help from Wayno on the datsun diesel site but there's very little traffic over there and the engine is running like a top now, so what I need is truck help more than motor help and I am hoping there will be more traffice here.   This is the truck:

 

Hmmm,

 

I tried to post a pic but I don't have permission, here's a link to it

 

 

https://lh5.googleusercontent.com/-y_n1SydzprA/TlHFXNTXrmI/AAAAAAAAAWY/oMuV1UehWM0/w800-h533-no/IMG_3368%2B%28Medium%29.JPG

 

 

[wayno]

That valve is a one way valve, it works one way, but not the other way, if you get it backwards, the brakes need to be pressed really hard to get them to work at all, now I am not exactly sure how that valve works, but it may have a limiter on how much vacuum it can pull at any RPM, once you removed that valve, you have full vacuum all the time, and when you rev it, it may even pull on the booster diaphragm harder, the result might be that the brakes are being activated by vacuum, the higher the rpms, the harder the brakes are being pulled on, as that  hose goes to a vacuum pump on the back of the alternator.

You should put that valve back on and see if you are able to move again, but it has to go on the correct way, I got mine backwards once, lucky no one was in front of me, the brakes didn't work very well.

I bet that when the engine is off/not running, it will move forward without issues if you are pointed down hill of course, start it up, and it might not move anymore.

It is easier to move a truck backwards with the brakes on, but it's not nearly as easy to move it forward.

Here's a new one for me.

 

I am slowly troubleshooting everything my 720 needs to get back on the road.  I just finished putting in a fresh engine and started working on the brakes.  Yesterday I was able to drive the truck forward in 1-3 gears and reverse.  The brakes were really bad so I started working on them.  

 

I bled all four brakes, no help.  Truck still goes both F and R.

 

Today I started checking the brake booster.  Bad valve between the vacuum source and the booster.  No vacuum is passing.  So I bypass the valve and hook the vacuum directly to the booster.  And then I backed up about ten feet.  The brakes work fine in this config.  Then I try to go forward and the truck stalls.  I start it and go forward again and this time as I let off the clutch, it begins to stall so I push the clutch back in.  Each time I try to go forward, the truck acts like the E-brake is on.  As I let off the clutch, RPMs fall until it stalls.  If I rev up, I can feel the car straining, as if I am trying to drive over a cinder block or push something heavy, and then the revs fall until the motor stalls.  

 

But not in reverse.  In reverse the truck acts just like normal.

 

Thinking that futzing with the brakes caused this I unplugged the vacuum from the booster.  No change.

Thinking that there was a wheel somehow stuck that could spin only in one direction I jacked the truck up and spun them by hand.  All wheels spin freely both ways.

 

I am thoroughly flummoxed.

[datzenmike]

The one way valve keeps vacuum inside the booster should your engine stall while driving. The stored vacuum would allow you to still have vacuum assist brakes BUT, you only get one or maybe two tries before the vacuum is exhausted and the brakes become very hard to apply. The diesel has a vacuum pump so I don't know how that affects this.

 

The booster diaphragm has vacuum on both sides of it. When you step on the brake, a valve opens and leaks vacuum out and air pressure leaking in pushes the diaphragm in the direction of peddle travel giving it an assist.

 

Thinking that there was a wheel somehow stuck that could spin only in one direction I jacked the truck up and spun them by hand. All wheels spin freely both ways.

 

I am thoroughly flummoxed.

 

Two things

 

1/ try spinning the wheels with the engine running

 

2/ Try letting the clutch out in neutral to eliminate the trans stuck in reverse.

[Mrs. Cold]

After sitting an hour, the truck moves forward again.  I think Wayno's speculation that the diaphragm was actualting the brake is correct and it was the higher RPMs doing it.  The reverse gear is so low that I just let off the clutch slowly to start moving, first gear is taller and needs some RPMs, I think the extra vacuum caused the brakes to grip.  Really weird. Off to find a working valve...

[Mrs. Cold]

Nah Wayno, I am a dude.  I came up with the name because of how much the SD-22 misses when it's cold.  Mrs. Cold...

[wayno]

Nah Wayno, I am a dude.  I came up with the name because of how much the SD-22 misses when it's cold.  Mrs. Cold...

Get a block heater for the winter, at 20 degrees it's not likely going to start, I see that all the time on diesel engines, the guy down the road was un-successful last winter when it got really cold(Ford), but mine started right up, I have a block heater, I just need to plug it in about a half hour ahead of time, or if you use it everyday, you can use a timer that turns it on while your still in bed.

[Mrs. Cold]

YAY!  It was the booster check valve.  I got one off of Amazon for a who knows what and plumbed it in.  The brakes work (OKish) and the truck will move in both directions.  I think my goofy check valve bypass setup put so much vacuum in the booster that it held the calipers shut even when my foot was off the pedal.  Not sure why it moved in reverse but maybe it's a much lower gear than 1st and it overpowered the brakes but the F gears didn't give enough torque.  

 

That's my story.  So, on to the next thing.

 

Wheels and tires.  Electrical. Legal.

[wayno]

All is good  :thumbup:

 

I don't know much about wheels, I run mostly 521/620 stock rims on my rigs with 620 hubcaps, I guess I am weird.

If I were you, I would try to find a Datsun diesel in a wrecking yard and raid it for some parts that are hard to find, all the diesel specific electrical components, a starter, the alternator with vacuum pump, that valve you just fixed, the radiator if yours looks iffy, you just cannot have enough extra Datsun diesel parts, oh and I would suggest getting the transmission also, or at least the front case and bearing cover, you don't want to start searching for one of them after yours has went bad.

[datzenmike]

YAY!  It was the booster check valve.  I got one off of Amazon for a who knows what and plumbed it in.  The brakes work (OKish) and the truck will move in both directions.  I think my goofy check valve bypass setup put so much vacuum in the booster that it held the calipers shut even when my foot was off the pedal.  Not sure why it moved in reverse but maybe it's a much lower gear than 1st and it overpowered the brakes but the F gears didn't give enough torque.  

 

That's my story.  So, on to the next thing.

 

Wheels and tires.  Electrical. Legal.

 

Probably leading and trailing brake shoes. If installed properly the shoes actually work better in one direction. (forward) You may notice most vehicles stop better going forward than backing up.