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Technical GM metric low drag calipers *FIXED*

Discussion in 'The Hokey Ass Message Board' started by Bullit68, Aug 12, 2020.

  1. WZ JUNK
    Joined: Apr 20, 2001
    Posts: 1,669

    WZ JUNK
    Member
    from Neosho, MO

    Yes, I have an adjustable proportioning valve. I have more than 1100 psi on the outlet side of valve with it all the way open.
     
  2. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,647

    gimpyshotrods
    ALLIANCE MEMBER

    Where and how are you measurimg pressure at the calipers?

    How old are the flexible hoses?
     
    VANDENPLAS likes this.
  3. WZ JUNK
    Joined: Apr 20, 2001
    Posts: 1,669

    WZ JUNK
    Member
    from Neosho, MO

    I recently replaced the hoses to the caliper with new braided stainless hoses. The 1100 psi is at the end of the hose at the banjo fitting where it attaches to the caliper. The 700 psi pressure reading of the caliper is at the bleed screw. I remove the bleed screw and attach my gauge line. The gauge has its own bleed screw. I bleed the system by various methods but I try to check the system by bleeding through a clear tubing to a jar so that I can visibly check the fluid for bubbles. I remove all bubbles until I have clear liquid.

    As a side note I have bought gallons of brake fluid as I have worked my way through this project. A quart at a time. I have bought more than I have ever bought in all my years of working on cars. This has by far been the most difficult problem I have ever worked on. I have built entire hot rods in the time I have worked trying to improve these brakes.

    I appreciate your time and any thoughts you may have.
     
  4. bobscogin
    Joined: Feb 8, 2007
    Posts: 1,761

    bobscogin
    Member

    Are you sure? The AFCO website description says they have "low drag" seals. Isn't that the source of your problem? From the website:
    " 2-1/2" bore steel GM metric caliper is designed to be a used as a NEW stock replacement caliper. The caliper features a stock appearing virgin plated caliper body, new ground 2-1/2" stainless steel piston, and low drag seals. Each caliper is assembled and pressure tested. The calipers have a 5.5" bolt spacing."
     
  5. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,647

    gimpyshotrods
    ALLIANCE MEMBER

    In a sealed hydraulic system, free of air, pressure will be equal at any point where it is being measured.
     
  6. WZ JUNK
    Joined: Apr 20, 2001
    Posts: 1,669

    WZ JUNK
    Member
    from Neosho, MO

    I isolated these components to take the readings. I took the readings at the bleed screw and then I disconnected the gauge and connected it again at the point where the brake line attaches to the caliper at the end of the flex line, so that I removed the caliper from the test. There is a difference of pressure between the connections for the gauge. I further isolated and tested each individual caliper. Interesting that the two calipers have slightly different readings. One caliper has more of a drop than the other. There are no fluid leaks.

    It seems that I have narrowed it to two possibilities. There could be air trapped somewhere in the caliper, that I can not get out. (the best answer) There is something pliable inside this system that is expanding like a balloon, that is allowing the pressure to change. My guess is the air but I do not know what else to do. I have removed the caliper, clamped the piston in place, and rotated the caliper in my hand while I bled the system. The bleed screws are pointed up but I wanted to be sure that I tried all possibilities. I have also tapped the caliper with a hammer while doing this.

    I bet you are starting to see why I am so frustrated. I have been trying to improve the brakes on this car for the last two years. Currently the original 4 wheel shoe type brakes were far superior to the 4 wheel power disc brakes I have now.
     
  7. southcross2631
    Joined: Jan 20, 2013
    Posts: 4,007

    southcross2631
    Member

    Gimpyshotrods you need to lighten up .My car is an 1/8 th mile nostalgic drag car. It was a bit of humor . I see the need for the best brakes you can get for street cars .
    With all of the stupid distracted drivers out there you need to be able to stop as quick as the big tires and little tire combo's that most street rods have.
    I don't see the need for the latest 6 piston caliper's and carbon fiber rotors on the front of a car that has a 4 inch wide tire on the front . Most hot rods should be be rear biased to take advantage of the larger contact patch that is on the ground.
    I am glad you live in a rich neighborhood that has those types of cars. Some of us are not that unfortunate.
     
  8. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 1,323

    Elcohaulic
    Member

    I need good brakes and handling because of our terrain. We have the steepest streets in the country here. It is a lot of fun driving here.
     

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    Mimilan, Tman, scotty t and 1 other person like this.
  9. streetrodder71
    Joined: Jan 15, 2015
    Posts: 796

    streetrodder71

    If you are using a under floor mounted master cylinder you will need a 2 pound residual
    valve on front & a 10 pound residual on rear if using drums. Sounds like fluid is running back
    to master cylinder . Jerry
     
    1934coupe likes this.
  10. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,647

    gimpyshotrods
    ALLIANCE MEMBER

    Someone put a 1000-foot mountain, right in the middle of our city!

    A >30% grade is considered normal here. twin-peaks-san-francisco-city-view.jpeg
     
  11. Jibs
    Joined: May 19, 2006
    Posts: 1,525

    Jibs
    Member

    Thats nice, but sometimes we get alot of snow .
     
  12. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,647

    gimpyshotrods
    ALLIANCE MEMBER

    Every so often, we get some too, and a whole mess of people die.
     
  13. Jibs
    Joined: May 19, 2006
    Posts: 1,525

    Jibs
    Member

    I'll bet, we have had snow here most winters, but we still have people that can't drive in it.
     
  14. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,263

    Atwater Mike
    ALLIANCE MEMBER

    This is like printed ping pong.
     
    fordflambe and scotty t like this.
  15. Mimilan
    Joined: Jun 13, 2019
    Posts: 643

    Mimilan
    Member

    Amateurs ^^^^
    Check out Baldwin St , Dunedin NZ [where brakes rule over horsepower]
    upload_2020-8-15_10-11-36.png

    upload_2020-8-15_10-13-20.png
     
  16. gene-koning
    Joined: Oct 28, 2016
    Posts: 2,025

    gene-koning
    Member

    The only hot rod I ever built that had crappy brakes had GM metric calipers and a "Corvette" master cylinder. I'll bet I know why those brakes were so bad now! Gene
     
  17. bobscogin
    Joined: Feb 8, 2007
    Posts: 1,761

    bobscogin
    Member

    The photos of terrain are scenic, but getting back to calipers, has anyone used the AFCO calipers and found them to work better than the GM metric calipers with regard to reducing the pedal travel? The AFCO website states that they have "low drag seals" which is exactly the problem with the GM calipers.

    Bob
     
  18. southcross2631
    Joined: Jan 20, 2013
    Posts: 4,007

    southcross2631
    Member

    They worked good on s dirt track car.
     
  19. lostone
    Joined: Oct 13, 2013
    Posts: 1,358

    lostone
    Member
    from kansas

    Ok I've got to ask this........

    Do you have the calipers on the correct sides? Are the bleed screws on top and the hoses on the bottom ?

    Can't tell you how many times I've seen this...
     
  20. Bullit68
    Joined: Sep 16, 2009
    Posts: 107

    Bullit68
    Member
    from Verona, PA

    To answer your questions... yes, yes, and yes. I ordered the Afco calipers, should be trying one in a couple days
     
  21. earlymopar
    Joined: Feb 26, 2007
    Posts: 1,242

    earlymopar
    Member

  22. bobscogin
    Joined: Feb 8, 2007
    Posts: 1,761

    bobscogin
    Member

    The master cylinder in the link you posted seems to be the type with the shallow pushrod bore intended to be used with a vacuum booster, not a manual brake application.

    B
     
  23. earlymopar
    Joined: Feb 26, 2007
    Posts: 1,242

    earlymopar
    Member

    Thanks Bob. Interesting. I copied the link provided by "Mimilan". I'll try the other link to see if they were swapped.
     
  24. 6sally6
    Joined: Feb 16, 2014
    Posts: 1,263

    6sally6
    Member

    I had S-10 calipers and a FoMoCo dual bowl MC for a Maverick. Never had much pedal. That puppy would go almost to the floor no matter what I did. One pump gave me a little more pedal but......panic stops don't allow time for a "pump-up".
    Once the guyz on here explained about the "step calipers" GM has I switched to a GM MC and things got a WHOLE lot better. Stops good.......good pedal......no more puckered butt hole with panic stops and 'mouth-breathing-millienial texters'! (sorry...just couldn't resist)
    Short answer.....swap MC's to match calipers.
    6sally6
     
    Mimilan likes this.
  25. earlymopar
    Joined: Feb 26, 2007
    Posts: 1,242

    earlymopar
    Member

    From what I can see, they all have a shallow countersunk feature which seems to tie into the comments I've read where users worry about the push rod falling out. Some users have gotten creative to provide a retainer of sorts for their applications. If you're aware of a master with a deeper recess, please provide a picture or link.

    Thanks,

    - EM
     
  26. 1934coupe
    Joined: Feb 22, 2007
    Posts: 4,122

    1934coupe
    Member

    I think Streetrodder is on to something. Disk brakes have a residual check valve in the master to allow fluid to keep some pressure on the pads. Drum brakes do not have said valve allowing the return spring to pull the shoes back and the fluid goes back towards MC. You stated:

    "I bet you are starting to see why I am so frustrated. I have been trying to improve the brakes on this car for the last two years. Currently the original 4 wheel shoe type brakes were far superior to the 4 wheel power disc brakes I have now."

    Did you use a disk brake MC or the one that was on there when it had drum brakes.

    Pat
     
  27. Jeff J
    Joined: Mar 15, 2007
    Posts: 950

    Jeff J
    Member

    So what your saying if you have the GM disc metric brakes on front and the GM disc rear brakes with e-brakes, that the bore size of the master should be ? As I have a cast iron master cylinder on it ! and not a good pedal !
    Cardone Select 13-1923 New Brake Master Cylinder is the one you recommending
     
  28. bobscogin
    Joined: Feb 8, 2007
    Posts: 1,761

    bobscogin
    Member

    To my surprise, my research shows the same thing. That's sure not typical. I'd find a way to positively retain the pushrod if I was using that mc.
     
  29. bobscogin
    Joined: Feb 8, 2007
    Posts: 1,761

    bobscogin
    Member

    This is just a thought for consideration, but could the low drag seal's retraction of the piston to a greater degree than a conventional seal be overcome by using a 10 psi drum brake residual pressure valve instead of the 2 psi often used for disk brakes? Perhaps it would retain the piston/pad much closer to the rotor and reduce the fluid volume required in the first pedal stroke?

    Bob
     
    1934coupe likes this.
  30. Bullit68
    Joined: Sep 16, 2009
    Posts: 107

    Bullit68
    Member
    from Verona, PA

    Okay, here’s the update! The AFCO calipers mentioned earlier ARE the standard/regular style caliper. A little more expensive than the auto parts replacements, HOWEVER, they have solved the braking issue! These have the correct style seals and pistons to run with my style master cylinder (Ford). You can see the difference in the brake pad drag on the rotors in the pictures. “Low-drag” caliper has almost. .020 and AFCO has almost nothing! Thanks to all again, much better braking now. And thanks to gimpyshotrods for the help!
    511A88CE-5D0A-4A95-85B2-1D23D408ECE9.jpeg 08F64654-D41B-4801-9C34-34DDBEE55EF3.jpeg
     
    Last edited: Aug 22, 2020

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