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    Results 1 to 7 of 7
    1. #1
      Join Date
      Aug 2022
      Posts
      1

      Brake conversion concerns

      I'm not sure if I'm posting in the right forum or not. I have an 85 El Camino that just came back from a local rod shop. They installed a Quick Performance 9" rear with their disc brake upgrade and S10 front spindles w/ 2 piston calipers along with UMI's Power Tour suspension kit. They also installed a Wilwood master cylinder (7/8" bore) and a 4 wheel disc brake proportioning valve. The problem is brake pedal travel. It's 2-3", almost to the floor. It's way more than the original set-up, which was disc/drum. The shop said they had a hard time with the brakes. They said that QP's brackets did not mount the caliper with the bleeder screw at the top. They claimed to have removed the caliper from the bracket, bled it and remounted the caliper. I was told they spoke with a tech at Wilwood who recommended a drop from the 1 1/8" bore MC to 7/8". Less volume=longer stroke but higher pressure. I was told mine is 1200psi. I've read systems need 600-1000 psi to operate properly. Although I think what I've read concerns manual brake systems because they talk about pedal pressure, pedal ratio and bore size equaling system pressure. Mine is vacuum assisted and I'm not sure how much that plays into the equation. Engine has 12" vacuum. The brakes work fine although I have not done any panic stops. I just don't like that much travel! Adding disc brakes to the system that requires more fluid volume and then decreasing MC bore size, just seems opposite of logic! Am I missing something? Is my logic flawed? This project has been off the road for 7 years for various reasons and, now that it's driveable, I want to get out and enjoy it. I just don't feel comfortable. So, I'm hoping to find someone to help me. Do I have a wrong size MC? Is the system OK and I just need to get used to it? Thank you for any assistance.



    2. #2
      Join Date
      Nov 2006
      Location
      Mountain Springs, Texas
      Posts
      4,489
      Country Flag: United States
      7/8 master with a booster is small. That is a manual master cylinder. 12” of vacuum is a little low for power brakes, which is maybe why they undersized the master cylinder.

      Sounds like bandaids on bandaids imho. With 12” of vacuum you might want to consider hydroboost.

      Those Cadillac metric rear calipers can be a major PITA and the caliper does need to be tilted to bleed.

      Hopefully one of the resident experts will chime in.

      Don
      1969 Camaro - LSA 6L90E AME sub/IRS
      1957 Buick Estate Wagon
      1959 El Camino - Ironworks frame
      1956 Cameo - full C5 suspension/drivetrain
      1959 Apache Fleetside

    3. #3
      Join Date
      Nov 2000
      Location
      O-town
      Posts
      4,281
      Country Flag: United States
      You could always call Tobin at Kore3 explain to him and hear his recommendations!

      And like Don said PITA metric calipers! M/C issues,

      I can't remember about the vacuum, it's been awhile since I had a car running, I thought 12 was good, but what do I know anymore!
      Steve68- 1968 Camaro SS LSX T56, 12bolt 3:90's, 18" Fikse Profil 13s, Deep Fathom Green paint, Spearcos, just bunch of old junk because another member said so, LOL



      70 Nova SS street/drag 454, T400, 3:55, ugly!

    4. #4
      Join Date
      May 2005
      Posts
      408

      Brakes

      dhutton,
      Steve68 is correct call Tobin and explain what set up you have.
      He will advise you on the safest way to correct your problem.
      When I've had brake issues he is the guy I call. Had an issue
      on my Camaro years ago banging my head until I discussed my
      issue with Tobin.So After I explained my issue he advised me what
      I needed to do. So I gave him my credit card number and said send
      me what I need His response was wait and I will get you a price on
      all the parts you need I said no you don't understand I know you will
      do the right thing to get my car to brake straight when I am stopping.
      Send me the parts.
      Sure enough I got the parts and installed them per our conversation.
      Brakes work perfect. GM 13" Zo6 on front and 12" GM rotors on the rear.
      Billy
      Last edited by morrisclassic; 08-07-2022 at 02:20 PM. Reason: adding text

    5. #5
      Join Date
      Nov 2000
      Location
      O-town
      Posts
      4,281
      Country Flag: United States
      Quote Originally Posted by morrisclassic View Post
      dhutton,
      Steve68 is correct call Tobin and explain what set up you have.
      He will advise you on the safest way to correct your problem.
      When I've had brake issues he is the guy I call. Had an issue
      on my Camaro years ago banging my head until I discussed my
      issue with Tobin.So After I explained my issue he advised me what
      I needed to do. So I gave him my credit card number and said send
      me what I need His response was wait and I will get you a price on
      all the parts you need I said no you don't understand I know you will
      do the right thing to get my car to brake straight when I am stopping.
      Send me the parts.
      Sure enough I got the parts and installed them per our conversation.
      Brakes work perfect. GM 13" Zo6 on front and 12" GM rotors on the rear.
      Billy
      Damn I feel half way smart! I actually need to call him myself,
      Steve68- 1968 Camaro SS LSX T56, 12bolt 3:90's, 18" Fikse Profil 13s, Deep Fathom Green paint, Spearcos, just bunch of old junk because another member said so, LOL



      70 Nova SS street/drag 454, T400, 3:55, ugly!

    6. #6
      Join Date
      Sep 2014
      Location
      Sun City West, AZ
      Posts
      672
      Country Flag: United States
      You might want to try installing a vacuum reservoir. The vacuum at 12 is low. Are you running a radical cam? If not, you might have a vacuum leak somewhere.

      The stock C4 Corvettes come with a 7/8" bore master cylinders. I ran the factory master cylinder with my Baer 6-piston calipers on all 4 corners and I never had any issues until the plastic pressure sensor on the master cylinder start leaking about 2 years ago. Then I replaced the master cylinder with the usual Chinese junk from Summit Racing and Corvette Central. Each master cylinder had different issues that caused either air get into the system or put enough line pressure on the front brakes to cause them to drag. After 4 junk master cylinders and 18 months later, Baer, and they were a big help and they convinced me to convert. They were very patient with me during that time and I should have taken their advice from the get-go and converted to a Baer master cylinder 2 years ago. I converted to a Baer master cylinder with a 15/16" bore and that solved all my problems.

      Do both rear calipers have the bleed screws pointing down? Is it possible that you may have the calipers mounted on the wrong sides and maybe just swap sides? Its very possible that both calipers have an air pocket at the topside of the caliper which is probably causing a soft/spongy pedal thus increased pedal travel. You might have to convert the rear calipers with a Wilwood or Baer Kit in order to make it right.
      --
      Kenny Mitchell
      [email protected]

    7. #7
      Join Date
      Oct 2004
      Location
      IL/TN
      Posts
      909
      Country Flag: United States
      IMHO it is really time to stop talking about brakes in terms of how many pistons one has, it's bragging rights for those that don't want to do math.
      You MUST know your calipers" total piston area before you go any further.
      I see guys with more piston area in rear than front !! oh but they have Corvette brakes !!
      https://www.protouringf-body.com "doing what they say can't be done"





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