Jump to content

521 brake adjustment

Recommended Posts

I've read several of the posts on brakes here on the forum and still have a question about brake adjustment on my 521.  I installed all most brake components new  from the master cylinder down to the new shoes with exception of rear brake springs and front hard brake lines.  up to this point i've bench bled the master cylinder at least three time and the wheel cylinders more than that.  i've adjusted the brakes to a light drag and still i cannot get a good hard pedal.  initially the pedal feels good just sitting in the truck but when driven it takes some effort to get the truk to stop and the pedal goes almost to the floor.  At this point i'm beginning to suspect i have a bad "new" brake master cylinder but the one thing i didnt do was to adjust the shoes  as described in one of the posts i.e. adjust till hard to turn wheel step on brake hard and if the wheel can still be turned adjust some more , etc.  Is this important or is one try to adjust till hard to turn sufficient.  Its been a long time since i drove a vehicle with 4 wheel drums so i dont remember how good(bad) these things are supposed to be.  

Link to post
  • Replies 6
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

most masters on 521s work or they dont.  I seen them leak out in 5 years for the cheap ones which is only the parts left in around to use now. Taiwan or Chinese maybe Italy.

I would tighten all of the drums up till a slight drag. and maybe a little more in the back as these brakes dont adjust by themselves. Usually when you at half pedal the rears need to be adjusted. but dont want to much brake lock up in rear as most people dont push on the brakes any harder thinking they lockedup but really its only the rears untill the later leveling sensor was installed to appy pressure equally to the brakes from and rear(dual master)


Only other thing is when one puts a new brake master in you have it correct at the firewall and at the pedal. all the ones I installed I had to cut the threaded post as it was too long and engage the pedal too early  then maybe adjust it there a little but usually Icut the threaded rod with the nut on there then back the nut off to clean the threads.


also by the gas tank fuel line cheak the brake line that runs above the filler hose as dirt builds up there and over time rots out the steel brake line!! usually a stack of dirt there keeping it wet( I know this from experience)



I still have the stockbrake springs on there  but rockauto and partsgeek do still sell them if needed

Link to post

I make a drum brake adjustment till there is more drag than I can turn by hand. Get in truck and stab the brake several times. This will center the adjuster as most likely only one show moved and began rubbing. Repeat till all the slack is gone and the wheel just barely turns. On the rears, loosen the e brake first, do both sides then re adjust the e brake.


If replacing shoes, some linings are centered on the shoe and the forward and rear shoes are interchangeable. While some have the lining clocked closer to the wheel cylinder than the adjuster.... these have to be specifically identified as the leading and trailing shoes and placed properly on the backing plate. When properly installed the leading shoe gains from a servo action and the forward turning of the drum helps pull the shoe into the drum increasing the friction. If put on in reverse the vehicle will stop easier backing up. If one wheel is reversed the vehicle will pull to one side.

  • Like 2
Link to post

Mike and Hainz have pretty well covered it.  A spongy pedal is air in the line.  If the pedal goes down slowly while pressing hard on it, there's a leak. The leak could be at a bore seal in the m/c or wheel cyls and you won't know it since it will take some time for the fluid to build up in the cups.  If it leaks at a fitting, that's pretty easy to find.  

The pedal won't feel rock hard until the shoes have "bedded" themselves to the drums.  Back in the day, they would grind/sand the shoes to perfectly match the diameter of the drum.  They don't do that any more so it's common for the shoes to touch the drums at high points.  If the shoes has a larger radius than the drum, the ends of the shoes will touch first.  If the radius is smaller, the center will touch.  Until the shoes wear those high spots down, you won't get full contact with the drum......meaning you'll get better braking as they wear.  This is also why it's a bit better, as previously mentioned above, to make them rub when adjusting.  A bit of rub will wear down fairly quickly.  Too much rub can cause things to heat up too much.  Better to rub a bit and readjust in a few days, depending on how much you drive it.


The excess pedal travel is because the shoes are not close enough to the drums.  You have one piston in the m/c pushing fluid to 4 wheel cylinder pistons at the same time.  If even one set of shoes is not as close/tight as the others, you get more pedal travel.  If it's one of the fronts, you'll feel pulling to the other side(typically).  If it's one of the rears, you probably won't feel the pulling.  


The m/c push rod should be adjusted correctly too.  The specs are in the factory manual.  There's a height off the floor that gets set first. If a previous owner messed with it, it can be off.  With the pedal at the correct height, adjust the push rod so that it still has a small amount of play in it.  Basically, it should be slack.....but by the time you push the pedal itself....1/4"(ish)?...that slack should be gone.


Did you bench bleed the m/c?  You can do this in the truck.  The m/c needs to be level. Typically, you can jack up the rear of the truck to achieve this.  Disconnect the push rod from the brake pedal.  The brake pedal will NOT push the piston in the m/c all the way in to eliminate all the air.  I typically don't deal with the single reservoir m/c's. I don't think they have a bleed valve on them.  You have to have two people to do this.  One person needs to push in on the push rod while the other cracks open the brake line at the m/c.  Hold some rags around it since you need it to leak fluid out.  Once the push rod is completely bottomed out in the m/c, hold it there and have the brake line tightened up.  Do this two or three times.  Now bleed the rest of the system starting with the wheel cyl that is the farthest away....based on the length of the brake line.  On most vehicles. that's the back right(pass side), but on some, the rear splitter is on the pass side of the diff so that makes the driver's side the one that's actually the farthest away.



Link to post

If you're having trouble bleeding brakes, set the parking brake and try again.


These trucks don't have any kind of LSVP, do they? If so, those should be bled first.


Pedal travel is super important too (as Mike mentioned above). Too much and the brakes won't work well. Too little and they can pack up and lock up. If there is a power brake booster, make sure the pushrod inside the booster is adjusted properly. Most are, but if it's worth checking. Kind of a bitch to actually measure, but it can be done with simple tools.

Link to post

No booster on the 521 and a single master for all wheels.

NLSV were used on the 620 stating in '76

Link to post

Lots to do, i didnt adjust shoes then stab the brake pedal to center the adjuster and continue adjusting.  I'll try that next.  additionally i bled the mc on the vehicle but did it with the pushrod attached to the pedal arm.  will detach the pushrod from pedal arm and do over. incidently i did bench bleed the mc in a vise with a hose and tube back to the resevoir.  I did have to cut the pedal arm as mklotz77 indicated so that necessitated removing the mc once more.  


As an aside, my wifey was helping me bleed the system, i was at the pass side rear and had already bled the system into a bleed jar so she was going to get best pedal and i was going to crack the bleeder to make sure air was out of the system. we did one pass at right rear and told her  get the best pedal while i cracked the bleeder one last time.  she pumped it up, and before i could crack the bleeder i heard the pedal hit the floor and all the brake fluid exited the long line from the diverter through a hole in the line an onto the floor. This of course left absolutely no brakes at all on any corner.  i was really glad that line popped when it did, i could see running up on a vehicle, stop sign or whatever ,hitting the brake and nothing there, and with a single mc system the sequence of events would not end well.  all this prompted me to replace the hard lines and i just bought a roll of nicu and worked from that.  that stuff works well and flares with a standard flaring tool commonly available as a rental from parts stores.  just be aware, if you're tooling around in a single mc version of the 520/521 or early 620 and havnt replaced the hard lines particularly the long line from the diverter to the rear brakes and the brake hoses, you're living on borrowed time.  when those lines go they go in a way that gets you looking for a soft place to land and often there aint one.

  • Like 1
Link to post

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.