521 brake hydraulic problem

[DanielC]

The brakes on one of my 521 trucks were and are working good, but recently they have developed a problem.

The brake pedal gets soft, and has increasing travel, and I have to bleed the brakes about one a week, or more depending on how much I drive the truck. 

I am not losing any brake fluid.

I only need to bleed one wheel cylinder, and after about six or eight pumps of the brake pedal, the pedal is high and hard again.  I only need to bleed one wheel cylinder to get good results, and have worked my way around the trucks wheels, and the result is the same, a few pumps, bleeding the brakes and its good again.

I replaced the master cylinder with a Centric one, and I am still having this problem.

 

[datzenmike]

Does air bleed out?

 

Have you adjusted the shoes out properly? on all four wheels? then adjust the e brake? You want the shoes as close to the drums as possible but not excessively rubbing them.

 

 

[banzai510(hainz)]

I noticed if I have my wheeled turned one way the brakes are harder and pedal seems firmer near the top. Then straight feels looser. I dont know what this means besides a bind or soemthing.

 

What I have been doing is tighenen the brake up which helped then this happened what I mentioned above. I need to tighten up the rears but its harder with the lowering blocks in the way and I cant get one of the drums off. i TIGHTEN THEM NOW BY PULLING THE DRIMS OFF AND USE THE ADJUSTER THEN PLACE ALOT OF ANTISEIZE ON THE DRUM MATING SURFACE AND IF POSSIBLE DRILL AMD MAKE A THREADED JACKING SCREW HOLE..

 

Daniel,I  had a bad master that would pump up and not release and would have to hit a bleed screw to get home w/o cooking the brakes.

[DanielC]

Getting a response from L motor god and King rat in an hour is fantastic, thanks guys!

 

I do not know if any air bleeds out.  This is the bleeding cup I use,

and when I am in the truck, stepping on the brake pedal, I cannot watch the cup for air bubbles.  All I can see is more fluid in the cup, after I am out of the truck.  I only pump four strokes, and refill the fluid in the MC.

Brake shoes are adjusted good.  The method I use is to jack the wheel up on the wheel I am adjusting.  Tighten the star wheel adjuster, until I cannot turn the tire any more, either direction, this usually happens when the star wheel is tight.  Then I step on the brake pedal, hard, and see if I can spin the tire.  If yes, more star wheel tighten, and repeat stepping on brake pedal.  Then I back off the star wheel 9 clicks.  I know the factory service manual says back off 12 clicks, but there is no drag, and almost no scraping at 9 clicks.

The emergency brake cable does need some adjusting tighter.

 

Banzai, I would check the rubber hoses between the frame of the truck and the front brake wheel cylinders.

 

When I replaced the master cylinder, I used the old master cylinder push rod.   The push rod with the new master cylinder was too long.

 

This morning, after bleeding the brakes last night, I got a load of hay.  I stopped at the house to take this picture.

 

When I drove the truck down to the barn, the brake pedal was low again.   When I got out of the truck in the barn to open the feed room door, to unload the truck, I noticed the brake lights were on.   This truck uses a hydraulic pressure switch for the brake lights.

 

I am going to loosen the master cylinder push rod and see if that fixes my brake problem.

 

 

 

[banzai510(hainz)]

on most new brake masters I have to cut the threaded rod a half inch or so.. As they are too long. if installed this way it preloads (pushs the piston)

I will install the master to see how the rod hit the pedal and make a best guess cut so the couple fits thru the pdeal hole and releases correctly.

 

I wont swap the rods as this has happen with franks son and his nice 510 where he swapped a rod and and later fell out at a stop light smashsing his front end a and bluebird grill.

 

 

I would try a slight drag when adjusting the brake. try 3 or 4 clicks.

Really Im winging it!!!!!!!!!!!!!!

Edited August 6, 2018 by banzai510(hainz)

[thisismatt]

Was it doing the same thing before you changed the master cylinder?

[DanielC]

I just adjusted the brake master cylinder pushrod about a turn and a half looser, and bled the brakes again.  Time for a test drive.

 

Yes, this is a problem that started when I had the old master cylinder in the truck.  That is why I changed the master cylinder.

 

[thisismatt]

I feel like I have a similar issue, but I think I have a small leak...

[DanielC]

After a short test drive, lost brakes again.

I wish it was as simple as a fluid leak.

[datzenmike]

 

 

So becomes softer over short time, can be....

 

Air sucked back into the system. Master has to move more fluid just to compress this bubble. As this hydraulic system only has a single master, if this bubble is between the master and the junction with the stop lamp switch bleeding any wheel will remove it and restore the pedal firmness. 

 

 

 

 

 

[thisismatt]

There has to be a leak somewhere, or an inlet for air, but with no leaking fluid it would have to be acting like a check valve and only letting air in during pedal release, which seems unlikely.

[DanielC]

I bled the brakes again, and parked the truck, and did some work on another of my 521 trucks.  In the evening, I went to park Ratsun inside, and the brake lights were on again.  I disconnected the brake light switch.

I am thinking of taking the old master cylinder apart and inspecting it?

Remove wheels, and brake drums, and looking at wheel cylinders?

I looked at the Factory service manual, it had a possible cause as a plugged vent in the master cylinder cap?

 

Just bite the bullet and change over to a dual master cylinder, and put front disk brakes on it?

[wayno]

OK, this would be my guess on why the brake light keeps coming on, my thinking is that the brake pushrod is adjusted wrong, my guess is when you let off the pedal and walk to the rear the brake lights are off, but if you were to stand there a little while they would come on when the brake shoes all around settle, as they settle they push the brake fluid back into the reservoir, but if the pedal rod is adjusted wrong or is too long and it doesn't release then that brake fluid in the lines has no where to go, so it builds up pressure in the brake lines till the brake light pressure sensor turns the brake lights on again.

If the brake pedal doesn't have any play where the rod goes into the brake master then it is too tight, you have to have a small amount of rod play, not a lot, just enough so you know the rod is not pushing on the brake master piston.

I am not sure why you have a soft pedal, maybe it has something to do with the brake pedal rod being adjusted wrong, check the rod adjustment.

[DanielC]

I though brake master cylinder pushrod adjustment was off too.  I tried adjusting the push rod so that there was no pressure on the brake master cylinder, but it could be still not correct.

 

[DanielC]

I got a chance to work on Ratsun tonight.  I removed the brake line between the master cylinder, and junction block, made sure it was clear, and not partially clogged.

I also removed the stop light switch from the junction block, made sure it was clear, and tested it electrically, the switch is always on.  I need to replace it, or put a pedal actuated brake light switch on the truck.

Ratsun may be a drive around the farm truck for a while, until I can trust the brakes again.

 

[DanielC]

Today, I got a new hydraulic brake light switch.

I am lucky to have a good auto parts store nearby, Clackamas auto parts in Oregon City.   I called a local Nissan dealer this morning, to see if a brake light switch was available, it was not from Nissan.

 

Body parts on a 521 are measured using the imperial system, you know, inches.  I knew the flare nuts on a stock  521 are 3/8-24, and I was hoping that the brake line junction block hole for the brake light switch was just a standard 1/8 pipe.  It was.

 

This is the old brake light switch screwed into a 1/8 pipe "T"

 

The OEM brake light switch needed a 1" socket to remove it.

 

It needed a fair amount of torque to remove it, so a clamped the lower part of the brake line junction block in a pair of Vice grips.  I held the Vice grips in one hand, while unscrewing the old brake light switch with the other.

 

After removing the old brake light switch, I used a oil pressure switch socket to install the new switch.

 

This is the new brake light switch, installed.

 

Then I put the wires back on.

 

Brake lights on.

 

Brake lights off.

 

I did a very short test drive.  The brakes seemed OK.  It was late this evening, and traffic is basically gone, so i took Ratsun down to a local gas station, and got gas in it.  The brakes still seem to work normally.

 

It will take a few days of driving to see if I really got the brake hydraulic problem fixed.  I am now suspecting the old brake light switch was defective, and somehow letting air into the brake junction block, but not letting fluid leak out.

Edited August 16, 2018 by DanielC

[DanielC]

I have driven Ratsun 17 days, but not everyday, and 242 miles with out any more brake hydraulic problems.  I am pretty sure the problem was a bad brake light switch.

[hosestop@msn.com]

On the brake light subject , I also sell a 3/8 T  with 1/8 pipe NPT. pipe on one female side , a lot of hotrod guys delete the switch on pedal  , it can be on either front or rear  line ,does not care ..