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  •  » FYI Ford, Classic Mustang Tech Discussion
  •  » 68FB Front Disc Rear Drum Brakes Stay On after Press and Release

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    6/23/2021 6:12 AM  #1

    M1Mustang69
    I FYI 24-7

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    Registered: 5/17/2013
    Posts: 491

    68FB Front Disc Rear Drum Brakes Stay On after Press and Release

    Anyone ever experience the brake pedal staying on after it's been released? 

    The pedal itself flops down, so the arm that pushes to the booster drops back, but something keeps the brakes on......slower speeds you don't really experience, super seldom if at all, but when you hit or exceed 50mph give or take, the car could come to a complete stop if you let it.

    If this isnt strange enough, if you give the pedal a another quick snap (tap to press and release) it lets go.....also if you keep the throttle going or harder it will let go.

    The car is totally restored. 302 modded, T5 and the rest is restored to stock.

     

    6/23/2021 6:44 AM  #2

    Rufus68
    I FYI 24-7

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    From: Knoxville, TN
    Registered: 4/26/2017
    Posts: 889

    Re: 68FB Front Disc Rear Drum Brakes Stay On after Press and Release

    I understand that you're saying this only happens sometimes and only at speeds over 50 mph, right?

    Assuming so, remember that the drum brakes are "self energizing" to assist with braking pressure.  This is why the leading brake shoe is different than the trailing brake shoe.  When the brakes are applied, the rotational energy of the leading shoe actually applies more pressure to the trailing shoe.  These shoes ride on about six contact pads of the backing plate.  These contact pads need to be smooth so the shoes can move properly; however, these pads get grooves worn into them with usage.  The shoes can stick in these grooves and cause braking issues.

    It would be good for you to fully disassemble your rear brakes, clean and inspect everything especially the backing pads.  If you find grooves worn into the contact pads, weld these up and grind them back flat and smooth.  After reassembling your brakes go for another test drive.
     

    Last edited by Rufus68 (6/23/2021 8:44 AM)

    1968 T-code Coupe with a 302.  Nice car, no show stopper for sure, but I like it.
     

    6/23/2021 8:17 AM  #3

    MS
    Forum Originator

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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,256

    Re: 68FB Front Disc Rear Drum Brakes Stay On after Press and Release

    I assume all brake parts, including ALL hoses, are new.

    Old hoses swell internally, letting 1,000 psi braking pressure go through but not be able to release pressure under only return spring pressure.

    Stuck caliper pistons, or stuck caliper guides, can do the same thing.

    Booster output shaft could be adjusted too long, causing master cylinder to not release completely. This can happen even if the booster input shaft is slack at the pedal.  To test, back off the two master cylinder attaching nuts about 1/8” to see if brakes release. If they do, remove master cylinder and adjust output shaft shorter. Try to get 0.010” clearance between shaft and master cylinder piston once master cylinder is bolted up tight.

    And check the rear, as previously suggested for wear or improper placement of primary shoes.

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

    7/21/2021 7:33 PM  #4

    M1Mustang69
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    Registered: 5/17/2013
    Posts: 491

    Re: 68FB Front Disc Rear Drum Brakes Stay On after Press and Release

    ****UPDATE****

    I place a 1/16" WASHER  between the booster and MC to test the theory of the Booster adjustment (before attempting to rip the system apart which I'm sure you all know is a real POI given the lack of realestate)
    The 1/16" washer on each side yielded 0 results. I have 1/8" washers I can try next, but don't you feel/think that 1/16" should be effective?

       •Thread Starter
     

    7/22/2021 1:33 PM  #5

    MS
    Forum Originator

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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,256

    Re: 68FB Front Disc Rear Drum Brakes Stay On after Press and Release

    M1Mustang69 wrote:

    ****UPDATE****

    I place a 1/16" WASHER between the booster and MC to test the theory of the Booster adjustment (before attempting to rip the system apart which I'm sure you all know is a real POI given the lack of realestate)
    The 1/16" washer on each side yielded 0 results. I have 1/8" washers I can try next, but don't you feel/think that 1/16" should be effective?

    They would be effective unless the booster output shaft was improperly adjusted more than 1/16" too long.  You don't need washers at all.  Just back off the nut another turn.  The MC does not have to be tight against the booster, unless it gets so far away that the vacuum seal is opened between the MC and booster.  I would not drive it a long distance with the MC loosened like that but it should provide you with the answer of whether the output shaft is too long to allow the brakes to release.
     

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

     

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