Register now to get rid of these ads!

Technical disc brake swap issue..still got a soft pedal

Discussion in 'The Hokey Ass Message Board' started by Vincentnova, Dec 2, 2018.

  1. Happydaze
    Joined: Aug 21, 2009
    Posts: 742

    Happydaze
    ALLIANCE MEMBER

    You say you had 870psi previously. That won't stop you well. You installed a larger diameter master (so you get less pressure, all things being equal) and found you got worse braking. Yep, that's how it works!

    Please answer 31Vickys questions precisely.

    Chris
     
  2. paintman27
    Joined: Apr 23, 2011
    Posts: 256

    paintman27
    Member
    from new jersey

    I know this thread is getting a little old but.....I noticed the OP mentioned he bought a quick uptake MC #18M309 and said it didn't fit into the MC hole because he is using a small booster. I am also having the same trouble with my brakes and was wondering if anyone knows of a quick uptake MC that would fit into a 7" booster. I am assuming a corvette MC is not quick uptake, as that is what I am currently using. Am I correct on that assumption?
     
  3. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova


    sorry for the dealy ive been super busy

    i have a 8 inch dual diaphragm booster , those kind of universal booster you find with or for any kit .

    my pedal ratio is 4:1

    and no i didnt check the pressure with the new bigger bore MC ..but im sure it was less pressure than the 1 inch bore
     
  4. Garpo
    Joined: Jul 16, 2016
    Posts: 140

    Garpo

    Have seen shiny new boosters faulty. Can't remember if you have tested yours, but easy to do.
    Pump pedal about 10 times with engine off to remove any residual vacuum.
    Then, with foot firmly on the brake, start the engine.
    You should feel the engine vacuum pull the pedal down about 3/4 to 1" if it is working correctly.
    Garpo
     
  5. Garpo
    Joined: Jul 16, 2016
    Posts: 140

    Garpo

    Re pedal ratio. 4:1 usually gives a fairly heavy pedal. I usually check to see if pedal to floor travel gives full stroke of the master. Does your pedal go right to the floor when both front and rear bleeders are open?
    Ideally the pedal should hit the floor a tiny bit before the master cyl bottoms out. That avoids crushing the bottom seals and/or springs in the master, but ensures you get maximum fluid output. Good luck.
    Garpo
     
  6. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova

    yes the booster is good . i think bad booster will only give a very hard pedal when engine on ( if everything else works fine)

    in my situation , if i remove the master cylinder from the booster and test the pedal travel , the pedal goes and touch the floor
    with the master cylinder in place , engine on , pedal bottom out master cylinder, but dont touch the floor
     
  7. 6sally6
    Joined: Feb 16, 2014
    Posts: 573

    6sally6
    Member

    I tried the 18M309 MC on my Mustang (with Chevy calipers) and it cured my spongy/soft brake pedal!
    Sure glad I saw the post about the quick uptake MC. Had to modify the push rod and wallow out some holes but the MC works and stops like it was made for my Mustang!
    Thanx Kerrynzl
    6sally6
     
    Vincentnova likes this.
  8. ricko1966
    Joined: May 4, 2017
    Posts: 35

    ricko1966
    Member

    A couple of things come to mind.1. Residule pressure valves should be used if your not already using them.And try lifting the front of the car as high as you can try to make your brake bleeds the highest point in the system.Good luck.
     
    Vincentnova likes this.
  9. 55 phil
    Joined: Feb 5, 2019
    Posts: 21

    55 phil

    To bleed power brakes it's best to have the engine running
     
  10. Try it with leather sole shoes, rubber soles will always give soft pedal feeling.
     
    61Cruiser and Vincentnova like this.
  11. 54vicky
    Joined: Dec 13, 2011
    Posts: 1,037

    54vicky
    Member

    this is the best answer to his fking problem so far:D:D:D:D
     
  12. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,422

    V8 Bob
    ALLIANCE MEMBER

    It's really best to bleed without power assist. You want a low pressure/high volume of fluid to effectively purge air out of the system. Power assist can easily result in the fluid moving too fast and aeration.
     
    Vincentnova and 57 Fargo like this.
  13. dan griffin
    Joined: Dec 25, 2009
    Posts: 418

    dan griffin
    Member

    I put power brakes on my 55 Chevy wagon , stock drum brakes, duel master cyl. I had a pedal problem it was the push rod on the master cyl. side of the vacuum booster was to long. I shimmed the cyl. out from the booster and that took care of it.
     
  14. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova

    ive done this still the same
    im aware about the push lenght rod booster to MC
     
  15. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova

    residual valve should be used only if the master cylinder is lower than the caliper non ?
    my booster /master cylinder are on the fire wall
     
  16. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova

    my actual booster is a 8 inch and its very very close to the engine...and a uptake MC wont fit my booster ( the booster hole where the push rod is , is smaller)
    the booster i ve seen for uptake MC are bigger and wont fit...
     
  17. 54vicky
    Joined: Dec 13, 2011
    Posts: 1,037

    54vicky
    Member

    john lee and vicky gave you good advice if that fails take the foam rubber off of pedal already holy sh!t
     
  18. Brake fluid is not hard to out smart,
    It does exactly as the rules say And always.
    It requires a certain input in order to produce a given out put.

    On the other hand, people will put all kinds of shit together then wonder why the brake fluid isnt doing what they want.


    Two ways to go about this correctly
    Know your needed output and learn what the input needs to be and build it back to your foot

    Know your foot input limitations and build it out to wheels to get the output required to stop.

    Couple different ways to do it incorrect I guess, pick your poison.
     
    57 Fargo and 54vicky like this.
  19. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,363

    landseaandair
    Member
    from phoenix

    Dang I was hoping this thread would have a resolution. Seems every thread on the net like this doesn't either. Got the same problem and can't stand to have a car kick my ass.

    Chevy II, front disc swap.

    Pedal always the same, won't pump up.

    Pretty firm about half way down but can be muscled not too hard all the way to floor.

    70s (not metric) calipers.

    1" Corvette style master.

    Factory style combination valve.

    9" ford with drum brakes.

    Pinch rear hose, no change.

    Pinch either front hose, nice hard pedal.

    Tried a new master, bench bled carefully and even replaced one caliper and pads with different ones with no change.

    Thought caliper brackets may be holding calipers off square and rebounding but still same with them unpinned and free.

    Calipers not retracting, still snug on rotors with pedal up.

    Bleeders facing straight up.

    Bleeding normally and then flushing with long clear hose from each wheel to master wells.

    Flows great at each bleeder and pretty damn confident no trapped air.

    Because of fenderwell headers combination valve is just above master well height but doubt any air pockets since I am able to use my line loc as a check valve holding pressure at front wheels down line and pedal is rock hard.

    Last Chevy II was slapped together with no dodads whatsoever (residual, proportioning, etc.), just a Corvette master plumbed straight to the calipers and rear end and worked great. That's what I get for trying to be all fancy.

    DSCN9791.JPG DSCN9785.JPG
     
  20. ^^^ Not suggesting it is your problem; but I always find those aftermarket Corvette style masters a bit suspicious.
     
    Center of the Galaxie likes this.
  21. Yup, I've tossed one or two Corvette style master cylinders out and replaced with another style to correct a low pedal problem. I've also seen many on cars that seem to have good working brakes. For my money, I won't use a Corvette style master on anything.
     
  22. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,363

    landseaandair
    Member
    from phoenix

    Also, after bench bleeding second master, reinstalled with solid plugs in outlets and had hard high pedal with just over 1/8" movement at piston.
     
  23. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova


    Welcome to the club ..
     
  24. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova

    Its frustrating because every test feel like a low drag caliper problem

    What kind of master cylinder did you end up using?
     
  25. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,363

    landseaandair
    Member
    from phoenix

    Been messing with my car a bit. Thought the high mounted combination valve may have made it hard to bleed air. Removed it and put an adjustable proportioning valve under the master with new lines, still no change. Clamping hoses on front calipers always gives me pedal and can't be any air in them. Had someone get in and pump the pedal while I watched the brakes.

    Calipers visibly move. Not attached to the brackets and the gap between the caliper ear and bracket opens up .022" on each side with the pedal all the way to the floor. Put a solid steel square bar in the caliper minus pads and get .025" to .030" of movement in the end of the ears, biggest measurement with the line loc on and pumping it up fully. Not sure if that's normal? Don't have any other older GM disc cars to compare to right now.
     
  26. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,363

    landseaandair
    Member
    from phoenix

    Might have been mistaken on one detail that may make a difference. Talked to my buddy who's car we did the first of these swaps on a couple decades ago and whose brother has the last car I had and both had manual disc/drum 70s nova masters that should have built in residual pressure valves in the rear. He also said he has a dirt car with disc/drum that did the exact same thing and was night and day after adding an add on inline RPV in the rear circuit.
     
  27. I used a Raybestos MC-39177 on the last one I did. 37 Ford with some form of aftermarket booster and vette style master cyl. I exchanged it for the above mentioned master cylinder and it worked fine after that.
     
  28. Vincentnova
    Joined: Nov 6, 2015
    Posts: 49

    Vincentnova

    what bore is it ?
    i think these MC the rear bowl is for the front break correct ?
     
  29. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,363

    landseaandair
    Member
    from phoenix

    I know me and the O.P. both have tried just about every avenue by now but here's what I did today. Re did some lines and installed a 10# RPV in the rear and a 2# in the front just for the heck of it. Threw a known good caliper with a Teflon braided brake hose on with it blocked solid. Bled it upright 4 times just to make sure and no change in pedal which pretty much cancels out bad hoses. Also re bled all 4 corners before trying that.

    Seems as if there's too much initial travel in the pedal like it still has air in it but pedal doesn't pump up and I've been very thorough about bleeding it. Probably grasping for straws but wondering if it's something weird like the vent holes are drilled too far forward in the well and pistons have to sweep a bunch before building pressure? Too tired to mess with it right now.

    DSCN9800.JPG DSCN9803.JPG
    DSCN9794.JPG
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2020 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.