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Wierd Brakes

Discussion in 'The Hokey Ass Message Board' started by Beep, Dec 18, 2013.

  1. Disc front, drum rear. My brakes work fine. However they feel wierd sometimes. Here is what happens.

    First start up: brakes are fine, but a tad lower pedal than I like. About 60% down pedal.

    After about fifteen minutes of stop/start driving, the pedal begins to raise up some (about 5% down: just right to me).

    After further driving the pedal rises to 0% down (no slack whatsoever)
    until it is actually trying to stop the car as it drives.

    Wiered??? Any ideas?
  2. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,385

    from phoenix

    Gobs of threads on this issue. Check to confirm a little free play in pedal adjustment first, then see if return hole in bottom of master well is plugged.
  3. Something is heating up your brake fluid. Could be a line against a header or similar.

    Second scenario, residual pressure valve doesn't work properly. Not holding to the preset value correctly.
    Joined: Aug 10, 2006
    Posts: 2,260


    I recently had issues with a vacuum assist disc brake set up. Same deal as you described, It turned out to be a bad vacuum hose.
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  5. I'm going with heat related issue too on first guess.
    Expanding fluid due to heat sounds logical.

    It could be some sticky components in the system too.
  6. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,236


    Can't see just heat doing it.
    With the brake pedal at rest the master cyl piston is back far enough that the ports are open and fluid expansion will simply raise the level in the master cylinder.

    As a guess I would say you don't have the required free play at the pedal and the piston isn't coming back to open the port.
    As has been said...lots of threads on this with lots of info.
  7. I'll certainly check to make sure no lines are getting close to the exhaust although I don't think they do. The lines are in their orginal places and I don't have headers.

    The MC and Vac booster are brand new as are all the brake components.

    I know that the rod adjustment between the vac booster and the master MC are critical. I was told it needs about 1/16 to 1/8 clearance. I may have this too tight. The clearance bewteen the MC and foot pedal rod does have about 1/8+ slack.

    I'll let you know how it turns out.

  8. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,547

    V8 Bob

    The booster push rod should have a Maximum clearance of 1/16" with the master cylinder primary piston, and that's checked with full vacuum, or about 20" Hg. to the booster.
    The pedal has to be able to return farther than the booster will allow, so check this by simply disconnecting the push rod from the pedal.
    The pedal needs it's own return spring, as the booster internal springs are not designed to return pedals and linkage.
  9. tommyd
    Joined: Dec 10, 2010
    Posts: 11,325

    from South Indy

    THERE! I learned something new today. Thanks for that bit of info.:)
  10. czuch
    Joined: Sep 23, 2008
    Posts: 2,756

    from vail az

    Theres stuff here thats rock solid advice and things I didnt know.
    My addition would be the initial bench bleed. Didja push it all the way down first?
    I did that once ad I learned alot from there.
  11. Andy
    Joined: Nov 17, 2002
    Posts: 4,504


    I think you have a hole plugged on the bottom of the resovour or like others said, not enough push rod clearance.
  12. I would give it another 1/8" of rod adjustment clearance & see what happens. That should be close to fixing your issue as it isn't that severe. I've seen too-little clearance lock them up as tight as a clam's ass.

  13. Thanks guys. I did a real quick job of backing off on the rod between the booster and MC "about" another 3 turns "in" to shorten it some. I did not have time nor the room to take real measurements. That quick job did help a lot, but it still does it some. Tonight I will pull the MC off to get it out of the way, take carefull measurements and set the rod with a dead on 1/16th clearance. Hopefully that should do it. I have been wanting to swap out the booster anyway. It is 7" and I have a new 8" to replace it with. I'll do both at the same time.

    I already have a return spring on the pedal. I learned that a while back. Thanks though.

    One more question though. Someone mentioned "residual" valves. My set up does not have these. My set up has the vac booster, then the MC, then the proportioning valve. The fluid lines lead from the MC (front line) to the proportioning valve for the front brakes and the back line leads to the proportioning valve for the rear brakes. I have checked the instructions to be positive that all this is in fact hooked up correctly. All is good there.

    Is it required to have these "residual valves? If yes, is there a difference between the front and back valves.

    Thanks guys, I love this site more every day.
    Last edited: Dec 19, 2013
  14. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    from Phoenix AZ

    Disk front under floor Mc a 2psi residial, friewall MC no residial needed. Rear either case with drums use a 10PSI residal.
  15. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,547

    V8 Bob

    Make sure you have full (about 20") vacuum before you adjust the booster push rod.
    You haven't described your vehicle or location of the master cylinder. If mounted high, like the firewall, no residual is needed for disc brakes, but if frame mounted, a 2 lb rpv is suggested. (I can't verify they are required, as I've never worked on a production vehicle with disc brakes and a frame mounted master, cause' there are none U.S. made that I've ever seen. But I do use them on disc-braked rods with low mount masters with no issues.)
    If your rear drums are newer than mid '70s, you don't really need a residual, but adding a 10 lb rpv will help with pedal feel/travel, even on later drum brakes, and something I do recommend.
  16. Ragtop
    Joined: Nov 17, 2001
    Posts: 1,261


    I think you'll find your master cylinder has a built in residual check valve. A buddy of mine had exactly the same problem. He removed the line from the master and ran a drill through the valve. Problem went away.
  17. OK thanks guys. More information for you.
    *The vehicle is a 1959 Ford Ranchero and the MC is mounted on the fire wall.
    *Rear brakes are stock (all new parts) drum brakes.
    *The front brakes are a conversion kit to disc.
    *This kit came with a 7" booster. I just changed it tonight to an 8".
    * I checked for heat issues on the lines. There are none. And no leaks.
    *I do not have any residual valves on the system.

    Tonight I removed the MC and measured as best I could. I thought I had exactly 1/16 inch clearance between the booster rod and MC recieving sleeve. It is hard to tell because you cannot see inside once you install the MC.

    Then I tried it out. Once the engine is running for a while and the brakes have been working for a while, the same thing occured only worse. They want to lock up. I then backed off on the rod quite a bit (about 3/16 inch) and that made it good exept I think I went a tad too far. I now have good brakes, but low pedal at first. Maybe 1/2 to 3/4 inch from the floor. As you drive a while (maybe 7 to 8 minutes) the pedal then comes up to where it is about right. Maybe 1 1/4 inch from the floor. I think I will lengthen the rod slightly one more time (maybe 1/16 inch) and see what happens. Damn this thing is touchy. A very small adjustment makes a BIG difference.

    The good parts is that it stops on a dime.

    P.S. what does a residual valve do?
  18. caton462
    Joined: Jul 17, 2013
    Posts: 176


    The residual valve keeps some pressure in the drum part of the system to keep the slave cups spread in their bores. Most of my customers who describe your issue turn out to be master cylinders with modern caps that they have overfilled with fluid. A little air space is required for expansion.
  19. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,547

    V8 Bob

    Drum brake residuals keep air from entering the wheel cylinders during brake release. Your older drum brakes do need them, so check for an internal residual in the rear brake port of the master cylinder, and if none install a 10 lb rpv in line to the rears.
  20. OK, thanks guys. I am getting real close to having this puppy whipped. I will order a residual valve tomorrow.

    Caton 462. I checked and yes, the MV is full to the top. I will drain some out tomorrow.
  21. good/nice thread guys....
  22. dadseh
    Joined: May 13, 2001
    Posts: 527


    There is no good reason for the pedal to "come good" as you drive... you have brake drag due to the booster holding on. To check if your booster is set right get a straighted out paper clip and probe down into the compensating holes in the MC. This is done after you assemble to booster of course.
    If it feels solid metalic its all good, if it feels spongy then the cups are covering the holes.

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