Brakes still randomly build pressure to cause undrivable dragging, everything replace

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There was a thread, not too long ago, somebody had a similar problem. turned out he was missing the brake pedal return spring, so his brake pedal never came all the way up...probably a silly question since you've done all this work, but, do you have a brake pedal return spring?
 
Quazii said:
The push rod is only slightly too long. Should have between 0.12in and 0.23in freeplay ( distance between where pedal rests normally to where it touches the push rod, measured at the foot pad). Hard to explain but the FSM should show it as my Haynes does. Otherwise just back it off a quarter turn or so. Your brakes are not seizing quickly so it's not off by much. If you can set per factory spec. it would be best as you would eliminate it from the equation. You may or may not have to pump them at this point. Mine was pretty off so when I backed the push rod to where it needed to be pumping for me became necessary. If you back off to far pumping will always be necessary even after proper wheel cylinder adjustment

Now it's time to adjust the wheel cylinders. After you back up and pull forward again (slightly applying the brakes) they will need readjusting. Dragging them helps center the shoes. Basically what happens after the first few adjustments is the dragging you hear while adjusting is only from one side of the shoe. As you center the shoe the side that was dragging slightly moves away from the wheel (now it's not dragging) and the side not initially dragging has moved closer to the wheel. eventually these distances become even and the shoe has a tight tolerance and will function properly. Did I mention it's tedious?;) The bright side is it will get there.

I hope this clears it up a bit. I remember how frustrated I was till talking to Mark. My brakes havn't seized since.

Remember: adjust then back up/pull forward
repeat
repeat...be patient...continue till it gets there
Keep in mind it's just one small master cylinder trying to feed eight wheel cylinders. That makes for tight tolerances.

Hope this helps
Chris
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Chris finally has consistent brakes, thanks to talking to Mark and lots of patience. For the last two years all he has done has worked on trying to line out his brakes. Now he has the truck stopping without any surprises. He is so used the working on brakes that the free time is killing him. He will grab a wrench:wrench: and go hide under the truck and pretend!!:bang:

I didn't think these would ever be right and was looking to find the stuff to upgrade to disk and just have it all delivered to Chris. He would have to convert if I found all the parts, right? I would not drive his truck because of this. Now he happy because I will drive it, and we can put money into other parts of the truck. He is also happy that I trust the truck enough that I want to take an Old School vacation in the 40:steer:. The point is, with time and patience you will be able to fix this, too. It helps knowing that others have the same problem.

:princess:
 
Thought so. Well, along the same line...is there anything preventing the brake pedal from coming up all the way? Maybe even the stoplight switch? (just trying to think of any easy fix that doesn't involve the pushrod)


slickrock said:
yup have a return spring.
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I had this problem a few years ago when I did a complete teardown and buildup of my rig.
Brand new master, brakes, the works. It turned out to be the master cylinder.
Just because a part is new doesn't mean its good, escpecially when you get it from Schulks, Kragens, Vato Zone! I had to install two master cylinders to get it to work right. OEM won't let you down!
 
I may be reading this wrong, but when refering to the push rod, is that the rod attached to the brake pedal which pushes on the brake booster? there is also a rod between the booster and the master cylinder. that rod needs to be adjusted every time a master is replaced. If it is to long there is always pressure on the brakes. It's clearance is 0.1-0.5 mm (0.004-0.020 in.) without vacuum
 
Plowboy said:
I may be reading this wrong, but when refering to the push rod, is that the rod attached to the brake pedal which pushes on the brake booster? there is also a rod between the booster and the master cylinder. that rod needs to be adjusted every time a master is replaced. If it is to long there is always pressure on the brakes. It's clearance is 0.1-0.5 mm (0.004-0.020 in.) without vacuum
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push rod between the booster and master.

My first master was doing the same thing (why I replaced it).
 
Well thought welding the push rod to the right length would keep it from backing out so that isn’t/wasn’t the problem with my dragging brakes. The push rod is at correct length (FSM) but the brakes did it again today out of nowhere. I was testing my suspension to make sure my front DL was long enough after stretching it. I was putting one wheel on a large dirt mound at a construction site. I had it flexed out and pulled the parking brake, got out to check the DL and was happy it was fine. I climbed in and noticed the brakes seemed stiff. Pulled the parking brake and sure enough, the brakes started dragging. The push rod and the brake peddle are the correct length.

The only thing I can think is that it has to be air in the lines at this point.

When I got home (that was fun but it didn't totally lock up until I got in the garage), I pulled the vac. hose and tried to push the brakes. Hard as a rock. So I started bleeding brakes and of course the pressure let up. I even went as far as getting some RTV and put it around the vac line plug since it seemed loose. Thought maybe it could be loosing pressure there. I could swear I heard a new whistle as I was driving back and it seemed to be a bit louder before it happened. But then again, I could be reading at this point.

Last time this happened, I could put on the brakes, turn the engine off and I would get the correct foot feel from the peddle to indicate no vac leak from the booster and the Master is new.

More suggestions? I was planning a trip Saturday too.
 
Fast Eddy said:
Take the residual valve out of the front circuit at the master cylinder. I'll bet lunch and a brew on it.
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WHy would I want to do that? It is a OEM replacement 1973 drum brake master. I have 4 wheel drum brakes. I take that out and will not be able to keep pressure in my brakes. My first Master was doing the same thing.

Are you suggesting I need to clean out the valve? I replaced all my fluid and it was already a bit brownish after only a few weeks. think there is crud in there even after flusihing the system and it back up when I putthe new one in?
 
I had the same problem, turned out after much heartache to be the pushrod into the master cylinder.
Mine has no power assistance, but the theory should still be relevant.
My pushrod was straight with a rounded end, my new master cylinder had a taper inside inside the piston.. When I braked hard the pushrod was binding inside.
I ground the pushrod into a similar taper and voila, fixed.
Hope this helps, I feel your pain :)
BTW I'm still not really sure why it worked, but it did...
 
From what you describe, I had the same symptom that I battled for months. Tried a bunch of things, bled em' like 10 times.

Turned out to be some small debris that was caught up in the rear lines. At least that is what I assume because the final fix involved disconnecting the rear calipers and blowing them out with air. I also pumped some fluid through to purge the line.

Yes, I realize you have drums, but there could be a small amount of debris that is randomly working it's way into one of the more constricted parts of the line and blocking the flow. Then it sits after driving, debris settles down allowing them to work again...then it repeats at random.

With mine I had bled them so many times trying to purge, but the blowing out with compressed air seemed to be the trick. Of coarse, that is a bitch to get the air out and re fluid, but if you isolate one section at a time, or where you think it is most likely.

For me, it was only the rears and the driver side seemed the worst. I clamped the soft line with vice grips and disconnected it. Then I pulled the bleeder and blew air through where the line went in, so it blew out through the bleeder hole.

Once I reconnected and bled it out, it was 100% better and now after many miles I can say the that the problem has not returned. From the first time I drove it after the blowout, it was noticeably better and has not locked up or dragged since.

Hope this helps.
 
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Slickrock, when I had the same problem on my 77 drum/disc set up. I ended up flushing brake cleaner thru all my lines. The 77 has a proportioning valve that mounts to the bolt that hold the booster to the fire wall. I didn't know if there was any rubber parts inside this valve so didn't flush any solvent thru it. I pushed a rubber hose over the hard brake line and layed it in a container full of solvent. I than used a vacuum bleeder to suck the fluid from the fire wall area to the line that first connects to the wheel cyclinder. I did this knowing that I was going to change out all my soft rubber brake lines. I used the same method to do the front brakes also. I removed the line that went from the master to the proportioning valve and flushed it several time with brake fluid on the bench. After all the flushing was complete. I ended up flushing a quart or so of fluid thru the lines to remove any solvent that may have been left behind and than I replaced all the soft rubber brake lines. If your rig is not pulling to one side, the problem should be in a portion of the line before the block on the front axle housing or the block itself. My brakes most likely had the same fluid that the truck came with from the factory when I purchased it from the p.o. Since you did all this work already, take Fast Eddy's advice, pull the valve and rule out any possible problems one by one. If I'm not mistaking, there's not much between the master and the front brake axle block. I ended up buying a rebuild kit for my master that came with new residual valves and pistons for about $30.00.:wrench::wrench::wrench::wrench::wrench:

Good luck Dave
 
here is a thought, im new here take as you will. Do you have a header? is it possible the line is builing pressure because of the heat that is generated under the hood? there is a sheild above the rubber line from the frame to the axle ,has it been removed. it could be boiling there..causing pressure to rise then lock up.
 
303garage said:
here is a thought, im new here take as you will. Do you have a header? is it possible the line is builing pressure because of the heat that is generated under the hood? there is a sheild above the rubber line from the frame to the axle ,has it been removed. it could be boiling there..causing pressure to rise then lock up.
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You know, this got me thinking. I was only driving around town and not stopping that much and spent some time not using my brakes while testing the suspension.

When I pulled it into the garage, I went to bleed it and some steam did come off of the fluid when I openend the vac. cup that the fluid goes into. It was also a hundred degrees in the garage. No way whould the brake fluid be that hot.

So I read this post and took a look, sure enough, with my headers, there is less than an inch of space between the hard lines and the headers. COuld the heads heat the hard lines that much to cook the fluid inside?

I think I am going to disconnect the hard line and move it to the outside of my frame rail.

Normal town driving (in a 5 stoplight town) should not casue the brake fluid to be hot enough to steam when it is 100 degrees outside. Should it?

(Good first post)
 
slickrock said:
WHy would I want to do that? It is a OEM replacement 1973 drum brake master. I have 4 wheel drum brakes.
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Oops. I was assuming front disc. Nevermind...

slickrock said:
there is less than an inch of space between the hard lines and the headers
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This is surely a problem. DOT 3 boils at 400f and DOT 4 at 460f. What hard lines do you have on that side? My hard lines go from the MC over the engine on the firewall and down on the passenger side.
 
the reason i thought of this is because this weekend I'm installing my header. 6 into 1 maf(new style)
As Im pulling all the shields off, there's a reason why they are there in the first place. my rubber line is right there, too. so, I was going to move the lines also to prevent this from happening or burning thruogh the line for any reason. or make new shields. I've been lurkin here for about a month or too. trying to solve problems as i build before they happen to me. just bought me a 1975 Fj-40 . trying to learn what i can.
Greg
 
I would look at that master again, have you replaced it only once? is it an Aisin unit or aftermarket, I had the same issues you are mentioning and it turned out to be a crappy NEW rebuilt master. It may be worth your time to swap in a toyota new master to see if anything changes, if it doesn't you can just sell it on ebay. Good luck.
 
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