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Technical Rear brake lockup

Discussion in 'The Hokey Ass Message Board' started by rockable, Feb 9, 2020.

  1. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
    ALLIANCE MEMBER

    My 49 Chevy Wagon has a 57 Chevy rear end with stock drum brakes. The fronts are GM M calipers and there is a GM combination vale in the circuit. I'm almost certain the plumbing is correct because I corrected an error right after I bought the car. The car stops well but in a panic stop, the rears lock up and that is a scary proposition. Today, I had a close call while out for a drive.

    I will investigate this more thoroughly this coming week but, assuming all is plumbed correctly and the combination valve is ok, what other things could cause this? Adjustment of the rears? Pushrod from the booster to master cylinder too long? Help me get the possibilities fresh in my mind.
     
  2. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,018

    David Gersic
    Member
    from DeKalb, IL

    Fronts not grabbing or wrong proportioning valve for brake hardware.


    Sent from my iPad using The H.A.M.B. mobile app
     
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  3. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    The fronts are working and the GM combination valve is tried and true. I'm thinking that the rears may be heating up a bit and causing them to grab when pressure is applied.
     
  4. 6sally6
    Joined: Feb 16, 2014
    Posts: 900

    6sally6
    Member

    Buy an ADJUSTABLE proportioning valve. They have a knob to change the bias from front to back. (s'why it's called adjustable.
    6sally6
     
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  5. bschwoeble
    Joined: Oct 20, 2008
    Posts: 458

    bschwoeble
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    Agree with 6sally6. Factory proportioning valves are specific to the car it came off of.
     
  6. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    I'm familiar with them but I've never had to use one before. The 57 brakes aren't all that stout, so I will check things out. Maybe someone put oversize wheel cylinders in the rear?
     
  7. MAD MIKE
    Joined: Aug 1, 2009
    Posts: 402

    MAD MIKE
    Member
    from 94577

    Generic preset prop valves are only ideal for whatever vehicle they were based on.

    If your front brakes are 80's 'G' body 60mm metric calipers/10.5" discs, they were never paired with anything other than 9" drums on a framed car.
    Your '57 11" drums will rear bias the brake system. Even the 'G' wagons still sported the 9" drums.
    '57 rear wheel cylinders were 1" on the early cars, G bodies used 3/4"rear wheel cylinders. This again pushes the brake bias rearward.
    Get an adjustable prop valve and do some brake testing.
    The other thing is if you ever load up the car with people/cargo, you can adjust the brake bias rearward to improve loaded stopping performance. This always requires time/testing, but if you can load up the car with weights and readjust for every added 100lbs you can really improve the braking of your vehicle. Just mark the knob for each additional weight increase.
     
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  8. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    Thanks. That makes one more thing I have to change on this car. The list seems endless.
     
  9. lippy
    Joined: Sep 27, 2006
    Posts: 4,219

    lippy
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    from Ks

    You love it and you know it. :D I feel your pain. Lippy
     
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  10. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    Actually, I am ready to just drive this one for a while. Plus, I hate dealing with brake fluid. Its the best paint remover ever made.
     
  11. lippy
    Joined: Sep 27, 2006
    Posts: 4,219

    lippy
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    from Ks

    I hear you on the driving part. Hope you get it sorted out. Love your Buick. I have a 47. Lippy
     
    rockable likes this.
  12. This is the one I use, no braking problems. I have it wide open and have yet to adjust it. My rears are stock Ford, GM disc up front. prop valve.jpg
     
  13. ol-nobull
    Joined: Oct 16, 2013
    Posts: 1,330

    ol-nobull
    Member

    Hi. On m 46 Chevy with original brakes the rear drums would lock up in a panic stop. Turned out the car had the original flex line between line on frame and the line on rear end. Turned out this neopreme? line was deteriating on the inside and it had a hanging flap inside and the pressure of a panic stop jammed it down and sealed of the line under pressure. Brakes would have to sit 15 - 20 minutes for the pressure to seep off and release. new flex line cured the problem. Jimmie
     
  14. 4274SPEED
    Joined: Apr 7, 2017
    Posts: 11

    4274SPEED
    Member

    Had a similar problem once.I thought both rear wheels were locking but it turned out drivers side was . One of the brake shoe linings had become half unbounded from the shoe and would lock the drum when braked even slightly. On inspection everything looked fine. Threw me for a loop!
     
  15. 54vicky
    Joined: Dec 13, 2011
    Posts: 1,227

    54vicky
    Member

    another possibility is the back linings being contaminated think leaking cylinder, grease from leaking seals.probably not the case but costs nothing more than time to eliminate the possibility
     
  16. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    Being as stubborn as I am, I pulled the rear drums and decided to replace the shoes. Went to O'Reilly's and picked up a set of rear shoes, Brake Best p/n 55. After comparing them, this is what I found. The shoes that are presently on the car have a typical short front shoe and a longer rear shoe, as you would find on most self adjusting drum brakes. The replacement shoes I bought are all the same length. I'm definitely going to try this before making hydraulic changes.

    Does anyone know if the rear shoes for the drop center Chevy rear ends, manual adjust, are supposed to be the same length?
     
  17. Mimilan
    Joined: Jun 13, 2019
    Posts: 313

    Mimilan
    Member

    The 57 had narrower 1-3/4" shoes on the rear compared to 2" shoes on the front [even though they used the same sized 2" drums all round]
    Chevy stamped a different offset backing plate for the narrower rear drums so you cannot swap front shoes into the rear.
    They had a smaller friction surface on the front shoes.

    also

    Go and try your car on a slightly slippery surface and see which end locks up first.
    Less traction = Less G forces
    Less G forces = Less weight transfer onto the front.

    You might need a slightly smaller wheel cylinder to reduce clamping pressure.

    Manufacturers had the brakes proportioned for lower G force stopping ,then added a pressure limiting valve [proportioning valve] to prevent rear wheel lock up during "Oh F**** "moments.
    57 chevys do not have any proportioning valves, or residual valves in the system

    edit:
    If you proportioned the brakes correctly for panic stopping in dry conditions [as Road Racers do] , In the rain or other less ideal conditions you would have front wheel lock up Which is more dangerous

    you need to think from the tyre footprint backwards. It is at the limit of tyre adhesion where Fr/Rr bias problems rear it's ugly head.
     
    Last edited: Feb 12, 2020
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  18. Mimilan
    Joined: Jun 13, 2019
    Posts: 313

    Mimilan
    Member

    I hope this was a typo:)

    Using a smaller rear wheel cylinder decreases clamping pressure [it is the opposite effect to smaller M/C size]
    so going from 1" to 3/4" wheel cylinders pushes the brake bias forward.
    Which would be the desired result for the OP
     
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  19. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    You are absolutely correct. That is why I take these recommendations with a grain of salt. Thanks!
     
  20. The short and long shoe setup is a feature of self-energizing brakes rather than self-adjusting brakes. Depending on what the current prop valve was used on originally, a 9" self-energizing rear drum setup might put everything right with the world for you. The front disks should provide most of the stopping force for you anyway.

    Or, as already mentioned, there's always the adjustable proportioning valve solution.
     
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  21. rockable
    Joined: Dec 21, 2009
    Posts: 2,675

    rockable
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    I think I found the problem.

    0213200930.jpg
     
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  22. gene-koning
    Joined: Oct 28, 2016
    Posts: 1,539

    gene-koning
    Member

    Yep, that would do it. Been seeing a lot of these worn brake shoe pads wore out in the last few years, do you think this 70 year old stuff is finally catching up with us?

    Been seeing it on front disc brake brackets as well. I was helping a friend that was complaining about his disc brakes dragging. We pulled the calipers off and the steel housing the brake pads rested against had nearly a 1/4" dip in them. I welded them back up, filed them flat, the the disc brakes no longer dragged. Gene
     
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  23. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,545

    V8 Bob
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    Pretty sure '57s had an internal residual (check valve) in the master cylinder, like most if not all single system drum brake masters.
    Proportioning came along with front disc brakes in the mid '60s, although some drum/drum vehicles could have benefited with one.
    Why is front wheel lockup more dangerous than rear???
     
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  24. Mimilan
    Joined: Jun 13, 2019
    Posts: 313

    Mimilan
    Member

    If 55-57's had an internal residual (check valve) in the master cylinder , it must be very small. My M/C only has a single Flare Nut straight into the M/C casting.

    with the Brake lockup question! You need to talk to Road Racers.
    Those guys are at maximum braking at every corner entry. If they aren't, some young "hotshoe" will be passing them under the brake zone.
    Sure this is competition , but in the real world an emergency braking situation is the same [Road Racers get to practice it more often[​IMG]]
    Wheel lock-up is equal No brakes for that particular wheel [except minor tyre/road friction]
    Tyres don't work on co-efficient of friction they Grip which is why G forces exceeding 1. are possible without downforce.
    If for example you do a panic stop that causes 25% weight transfer to the front [on a 50/50 balanced car] The front brakes now have 75% braking and the rears only 25%.
    If the front locks up you lose 75% braking .

    As for the rears locking up first causing cars to spin out?
    The most efficient braking is not on a corner where a tyre is trying to control lateral acceleration and deceleration in the same tyre footprint .

    The saying is "Brake on the Straight before it's too Late"
     
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  25. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,545

    V8 Bob
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    The residual check valve is located internally and at the end of the bore on single system masters.
    On the street, fronts should always slide before the rears. Really best not to have any sliding for the best braking performance. :)
     
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  26. Driver50x
    Joined: May 5, 2014
    Posts: 38

    Driver50x
    Member

    Sometimes machining the brake drums can help. I’ve seen both out of round and tapered drums cause grabbing and locking up problems.
     
  27. Mimilan
    Joined: Jun 13, 2019
    Posts: 313

    Mimilan
    Member

    Your answer got me thinking/ worrying.

    I was given an almost new rebuilt / exchange M/C from a recently imported 55 Chevy. Because this particular car had modified brakes NZ law requires a dual M/C.
    This M/C was dismantled and there wasn't a residual check valve at all.
    Maybe for some reason, most rebuilt or new replacement master cylinders for these applications no longer have the residual valves.[ I don't know ]

    My car might or might not have one [I doubt if the M/C is original] but it works perfectly with no leaks or pedal travel issues.
     
  28. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,545

    V8 Bob
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    Below is a picture off the internet of a '55-'57 rebuild kit. The residual is located inside the big end of the spring on the right.
    Residuals are needed on vintage drum brakes to prevent air from entering past the wheel cylinder cups during release. They were used up until the mid '70s when front discs and wheel cylinder cup expanders made them obsolete. I still use them on newer drums to minimize pedal travel.
    [​IMG]
     
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  29. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 693

    Elcohaulic
    Member

    Bob, how do residual valves minimize brake travel?

    I might need them on my el Camino. The stock GM Disc / drum brakes work perfectly but the pedal travel is deep.. This is the first car I ever had that didn't have a lot of disc brake drag...

    Thank you
     
  30. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,545

    V8 Bob
    ALLIANCE MEMBER

    A residual keeps the line charged with fluid under very low pressure, minimizing the additional fluid needed to move the wheel cylinder piston(s), along with pedal travel.
     
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