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#1 northoak

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Posted 27 December 2016 - 06:33 PM

Hi all, After my 91 hardbody was stolen I recently bought a cheap 84 720 with some brake issues.  The rear brakes were locking up way to quick and skidding which seems to be a common problem.  I first tried to adjust the drums which barely helped.  I then bled the NLSV, rear brakes and front brakes and the problem was solved!

 

Unfortunately I drove around today and all was good until I got on the highway.  I had to brake suddenly and the problem returned.  The truck skidded a bit but luckily I came to a safe stop.  The whole way home the problem continued.  I had to engine brake hard and barely press the pedal to get home.  I do not see any visible leaks in the brake system but do see a very minor leak on the rubber boot of the clutch master cylinder.  Are the brake reservoir and clutch reservoir connected?  The fact that the problem went away and then returned implies a leak but I do not see one.  Any other ideas?

 

 Thanks alot in advance y'all, I am stoked to join the ratsun world.



#2 datzenmike

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Posted 27 December 2016 - 07:14 PM

Clutch and brake masters are not connected.

 

Most likely the NLSV. It is mounted with the front slightly higher than the rear. Inside is a steel ball. When hard braking, the ball rolls up hill and blocks fluid from getting to the rear brakes causing them to lock. As more cargo weight is added, the rear of the truck lowers effectively raising the front of the NLSV and it's harder for the ball to roll up hill and block the brake fluid. In effect, more weight increases tire friction to the road so more brake fluid pressure can be applied to the rear brakes without locking them.

 

Perhaps try tapping the NLSV with a hammer to loosen the ball. If this fixes the problem, then this is the problem.


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#3 northoak

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Posted 27 December 2016 - 07:27 PM

Thanks for the quick response, I gave it a few swift taps with a hammer but it did not seem to help.  I guess we can rule that ball out.  I've been snooping around old posts and also tried adjusting the free play in the pedal but no luck there either.



#4 flatcat19

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Posted 27 December 2016 - 07:49 PM

Are the brakes properly adjusted? Brake shoes, if too loose, will get themselves wedged on other hardware in the drum assembly.

You can't hang if your nuts haven't dropped...it's physics


#5 datzenmike

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Posted 27 December 2016 - 07:52 PM

Pedal free play is just a few mm or 1/16"... that's loads.

 

Next time the rears are near locked, loosen the front brake line on the master. The front is for the rears and the rear one is for the fronts. There will be an F and an R cast on the master. Just loosen slightly and if fluid spurts out then pressure is trapped in the rear lines. If it dribbles out then it's fine.

 

If there is brake pressure trapped then that's the master's fault.

 

 

Next time the rears are close to locking, loosen the brake line on a wheel cylinder or the NLSV slightly. If fluid spurts out then the pressure is trapped. This would be the MLSV's fault.


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#6 wayno

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Posted 27 December 2016 - 07:55 PM

Bleed the NLSV  again, if the problem goes away then it is likely a NLSV issue, if it comes back again then either get another NLSV from a wrecking yard or remove it completely, that might take some planning though depending on which NLSV you have, it's a complicated system on some years.


 

 

 


#7 northoak

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Posted 27 December 2016 - 08:16 PM

flatcat19: I believe the brakes drums are properly adjusted since they were working well yesterday

 

wayno: As far as bleeding the NSLV my friend I was with and I actually did that to see if it would help but it did not.  We actually were meeting up so they could help me flush the clutch so I had fluid, line, and tools in the back already.  There didn't seem to be air in the line.  We didn't try to bleed any of the wheels again.

 

KingRat: I will do that line check tomorrow forsure, do you think I need to drive around a bit in order for the pressure to build? They pretty much lock up first stop.  

 

I'm also planning on popping the drums off to make sure the shoes are moving properly, any idea what to look for in there?  I'm assuming both shoes should move outward when the pedal is pushed down and return all the way when the pedal is lifted?  Are their any common culprits within the drum asssembly?  I wondering if maybe all the pumping of bleeding the brakes broke some old crusty hardware loose that is now stuck again?

 

Thanks for all the info!



#8 flatcat19

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Posted 27 December 2016 - 08:59 PM

Loose brakes can work, sometimes. Same average for them to not work.

Adjust them tight. Remove it from your flowchart.


If you don't know off the top of your head with certainty they're probably not.

But that may also not be your only problem.

You can't hang if your nuts haven't dropped...it's physics


#9 wayno

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Posted 27 December 2016 - 09:57 PM

flatcat19: I believe the brakes drums are properly adjusted since they were working well yesterday

 

wayno: As far as bleeding the NSLV my friend I was with and I actually did that to see if it would help but it did not.  We actually were meeting up so they could help me flush the clutch so I had fluid, line, and tools in the back already.  There didn't seem to be air in the line.  We didn't try to bleed any of the wheels again.

 

KingRat: I will do that line check tomorrow forsure, do you think I need to drive around a bit in order for the pressure to build? They pretty much lock up first stop.  

 

I'm also planning on popping the drums off to make sure the shoes are moving properly, any idea what to look for in there?  I'm assuming both shoes should move outward when the pedal is pushed down and return all the way when the pedal is lifted?  Are their any common culprits within the drum asssembly?  I wondering if maybe all the pumping of bleeding the brakes broke some old crusty hardware loose that is now stuck again?

 

Thanks for all the info!

 

Keep in mind that the brake masters have residual valves, so that means that when you hit the brakes and let off the brake master is supposed to hold 10lbs of pressure on the rear drum brakes, and 2lbs of pressure on the front disc brakes, so don't expect the springs to pull back the rear brake shoes all the way right away, the drums stop the pistons in the wheel cylinders from moving that far out in the first place.

My 1986 chassis has this issue of having the rear brakes lock up the first few times I use the brakes in the morning, it happens so fast that the whole cab shudders, so I pump the brakes real hard when I am stopped at the stop sign and it usually goes away and acts normal till the next time/day I drive it, I adjusted the rear brakes a couple days ago and it fixed the slamming/shuttering, but the rear wheels still locked up the next time I drove it.

I didn't have this issue when I first built the truck, but it came on after sitting for long periods of time, sometimes even my e-brake sticks and I have to drop the clutch in reverse to break it loose, this e-brake issue has happened on two of my 720s that sit for long periods of time.


 

 

 


#10 kelowg

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Posted 28 December 2016 - 06:47 AM

Long shot,but i had the rear hose (axle to frame) fall apart internally. Causing intermittent blockage on rear brakes in turn causing them to drag so bad,had disconnect rears to get home(100 miles). If any dought of brake hoses' condition,REPLACE THEM. Had front hose burst half way over winding mt road,not fun.



#11 thisismatt

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Posted 28 December 2016 - 07:08 AM

Are they locking up with light pressure/short pedal travel?
I'm your huckleberry.

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#12 northoak

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Posted 28 December 2016 - 11:10 AM

Thisismatt: yes, they are locking up with light pressure/short travel



#13 northoak

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Posted 31 December 2016 - 12:13 AM

Ended up replacing a questionable looking wheel cylinder today with some luck.  So far they aren't locking up but I haven't gotten on the highway yet.  I'll post here when I know for sure that the problem is gone,  thanks for all the suggestions.



#14 flatcat19

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Posted 31 December 2016 - 12:16 AM

http://community.rat...brake-struggle/

This is a recently made thread. All suggestions you need/ advice is detailed in here.

Get after it. You'll be running-er, stopping in no time.

You can't hang if your nuts haven't dropped...it's physics