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Weak/spongy brakes


Rusty Dawg

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Howdy,

 

I replaced my brake pads in my 61 Datsun truck a few months back. I turned the drums, honed the brakes cylinders, replaced the caps, replaced the master cylinder and I have bleed the lines at the wheels at least 10 times on level ground as well as with the back jacked up and then with the front jacked up(1st rear passenger, then driver's rear, then driver's front and finally passenger's front).  I have adjusted the brakes to where I get a light drag.  I have also bleed the lines at the master cylinder and I still can't get firm brakes.  The brake pedal almost goes to the floor.  I need to pump it 3 times to get a firm brake pedal.  NO leaks anywhere and the reservoir is constant at the "full" line.  For the life of me, I can't think of anything else to do...can anyone enlighten me?

 

Regards

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Did the early ass trucks have a nslv valve?  And not that I think it matters but on non abs vehicles I am fairly certain you bleed from closest to furthest away in that order.

 

And when you replaced the master, what did you do for a push rod?  Any chance it it traveling too far before it starts doing its job?

 

I hope this helps.

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14 hours ago, bilzbobaggins said:

Did the early ass trucks have a nslv valve?  And not that I think it matters but on non abs vehicles I am fairly certain you bleed from closest to furthest away in that order.

 

And when you replaced the master, what did you do for a push rod?  Any chance it it traveling too far before it starts doing its job?

 

I hope this helps.

I did use the same rod that was on the old master.  That is something I thought about a few months ago, but forgot to try.  Maybe going with the adjustable rod that came with the new master cylinder will fix my problem.  That's has to be it🤗  Thanks for reminding this forgetful ole bird.

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13 hours ago, datzenmike said:

NLSV first used on the '76 620.

 

Did you pump 3 times to get pressure and then loosen the bleeder screw  on the wheel cylinders?

 

Did you bleed the new master?

 

 

I did pump the brakes 3 times to bleed at the wheels as well as at the master.  It was suggested that it could be the pushrod and I think bilzbobaggins is onto something.  Needless to say, it's been scary as hell going thru intersections knowing that I wouldn't have time to pump the brakes 3 times to get it to stop quickly if someone cut me off.  This forum rocks!

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Re: the adjustable master push rod.

 

You MUST have about 1/16" of play at the brake pedal to allow the master to return to it's rest position. If not pressure will be trapped in the system and added to until the brakes drag continually or are locked up. Push on the brake pedal with your thumb. How much slack or play is there before you feel resistance?

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Pedal free-play should give you a reading on if you have a push-rod problem. I think that Mike mentioned the necessity of having the master piston return to the fully retracted position and have a specified amount of free-play. There is a small hole in the top of the master cylinder bore just in front of the piston cup when it is in fully returned position. This allows any pressure that might be built up in the system downstream of the master to relieve back to the reservoir and balance the system. The little hole also allows the system to add fluid to the system when needed. What I'm saying is that the piston needs to be returning to the fully retracted position when the pedal in full return position. I'm assuming that you have all original or near so equipment in the system. 

 

Since refurbishing the hydro clutch and the brake system our 320 upgrading to dual circuit and rebuilding the clutch system on the 620 'Longbed' I've become partial to bleeding the system backward from the usual practice of bleeding from the master reservoir to each terminus of the system. I did those systems pumping fluid backward to the reservoir. Where you have to be careful doing it this way is pumping old fluid with debris back into the master if the upstream system is new and downstream is old.

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18 minutes ago, thisismatt said:

Having to pump multiple times would always suggest to me that there is still air in the lines.

I agree ^^^ or the brake pads are not adjusted correctly.

You want to make sure your drum brake pads are adjusted correct. Wayno posted some info few years ago on how to correctly adjust drum brakes for a 521. Should be the same for yours, I hope. Once the brake pedal is properly adjusted as mike said above, tighten the brake pads till the brake drum doesn't move,  then count 7 clicks when loosening the brake pads on each drum. This should center your pads inside the drum. I've done this ever time I adjust my 521 brakes and it's nice that all four corners work together.  

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Sound like you could have air in the lines or shoes too loose or a combination of the two.

Both have been mentioned above.

If you don't want to do the bleeding sequence you could have them pressure bled by anyone with a pressure bleeder and universal adapter for the master cylinder.

 

My sugggestions would be:

1. get some 3/8-24 speed bleeders for all 4 corners of the truck. Part number Sb3824 . if you like them a little longer get SB3824HD

   They work very well and help air from backing into the system through the hose or bleeder threads while bleeding.

the company also sells great silicone brake bleeder hose.  I made 4 bleeder jars one for each corner so I can just put the hoses an and go quickly from corner to corner.

 

2. Check the pushrod and make sure there is just a tiny bit of clearance in it, the minimum is tiny tiny bit of free movement of the pedal before it touches the pisoin in the master cylinder.

 

3. make sure you have an assistant who keeps the master cylinder reservoir at least 1/3 full, the reservoir is tiny and its easy to suck air.

 

4. bleed brakes in the following order the first go round. right rear/left rear/right front/left front. After you think all is clear Then pump pedal multiple times hard to break up bubbles into smaller foamy ones and bleedagain  using a lot of foot pressure to drive the fluid out fast using the same sequence.

 

5. Adjust the rear brake shoes up click by click until they wont turn by hand- barely. Hit the brakes hard multiple times to help center the shoe in the adjusters and drum.

Then readjust till scraping, Hit the brakes a couple more times solidly and then adjust till they wont turn. Then back the adjuster off until you can turn it by hand and hear a them barly touch as you tuern them- tiny bit scraping, That little bit will wear off quickly. This is as tight as you can run them without heating them up .

 

6. Now if they are not great bleed them again. Bleeding with brakes tight means more fluid gets pushed out the bleeder screw instead of moving the shoes outward.

 

Now You should have better than factory brakes!

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get the drums soem what tight when adjusting. bleed the brakes. see where the pedal lands when you hit the brakes. If still takes 3 pumps then I would get another 35$ Chinese made master and swap it in.

 

most times the threaded rod on the master need to be cut as they are too long and the pedal arms engages too soon thus putting pressure on the master

 

 

what I do on 521 shoes is i adjust them with the drum off. make sure shoe is center adjut most of the gap out till the drum barely goes on. then I adjust from the back side using the wheel

 

I never had to bleed a master. I just fill them up. pour the fluid and start at the right right pump the pedal into a plastic bottle with a clear tube till no bubbles. then go to the next wheel ect...... only its a pain with a small master resivoir one needs to get out and refill evey like 3 pumps unless you have somebody filling it for you.

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7 hours ago, datzenmike said:

Re: the adjustable master push rod.

 

You MUST have about 1/16" of play at the brake pedal to allow the master to return to it's rest position. If not pressure will be trapped in the system and added to until the brakes drag continually or are locked up. Push on the brake pedal with your thumb. How much slack or play is there before you feel resistance?

I would say the pedal travels 2" before I feel resistance when using my thumb.

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Yeah that's most of your stroke wasted. Pull the cotter pin out of, and remove the clevis pin. With the lock nut backed off this will allow you to turn the adjustment out. Get 1/16 to 1/8" of play, put the pin in, check one last time, and tighten the lock nut. DON'T FORGET THE COTTER PIN!!!!! 

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I installed the new pushrod and although it doesn't look cosmetically/symmetrically great, it works.  The brake pedal is now 1.5" to 2" higher/shorter than the clutch pedal.  She actually stops nicely now.  I think I will still adjust the brakes per 420n620's suggestion.

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1 hour ago, datzenmike said:

Yeah that's most of your stroke wasted. Pull the cotter pin out of, and remove the clevis pin. With the lock nut backed off this will allow you to turn the adjustment out. Get 1/16 to 1/8" of play, put the pin in, check one last time, and tighten the lock nut. DON'T FORGET THE COTTER PIN!!!!! 

DIDN'T FORGET THE COTTER PIN!!!😂

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