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1977-81 GM f-body styled rear disc brakes fab/mod/retrofit

Discussion in 'The Hokey Ass Message Board' started by torchmann, Mar 12, 2009.

  1. torchmann
    Joined: Feb 26, 2009
    Posts: 787

    torchmann
    BANNED
    from Omaha, Ne

    If you would like to use 77-81 gm Firebird or Camaro rear disc brakes on your rod here is some vital information to create your own kit from scratch.


    The 1977 eldorado parking brake calipers and other parts needed are available new without core from:
    TSM manufacturing http://tsmmfg.com

    They also have the newer style GM calipers, conversion kits, and parts for other makes.
    here is a diagram of the caliper bracket with dimensions to cut your own from plate that can use the FACTORY ORIGINAL firebird/camaro parts.
    also here is a pic of the original brackets from a 1981 10 bolt disc brake rear.

    [​IMG]
    [​IMG]

    I have the factory setup on my 71 firebird. It is setup as 4wheel manual disc brakes.
    My 71 had manual disc/drum from GM. There is no difference between the 2nd generation f-body 4wheel disc master cylinders and the disc/drum mastercylinders.
    There is a difference between manual and power assist mastercylinders.
    There is a difference in manual and power brake footfeeds
    A manual master cylinder needs a smaller driving piston to get more hydraulic force multiplication (pressure) to the brakes through the driven piston (wheel cylinder). It also needs more piston travel to transfer the same volume of fluid as a larger piston to the driven cylinder.
    I'll call the force on the driving piston from the brake pedal lever A
    The area of the driving piston is B
    The hydraulic pressure between the driving piston and driven piston is C
    The area of the driven piston is D
    The force upon the brake shoe by the piston D is E
    A/B=C C*D=E so... A/B*D=E
    The units of measure may be english metric or other but what is important is to stick with the same units throughout the calculations. I will be using fluid pressure measured in "pounds per square inch"
    mechanical pressure will be in pounds, and the area of the pistons will be in square inches.
    The hydraulic brake system pressure created will equal A/B
    100# of pedal pressure on a brake arm with a mechanical ratio of 10/1 yields 1000#'s of mechanical pressure.
    This pushing through the pushrod on a 2 square inch master cylinder piston creates 500 "pounds per square inch" or psi fluid line pressure. It is this way because the force on the piston is 1000# total pressure divided by the square inches of the piston to get the line pressure.
    this line pressure pushing on 2-4 square inch pistons in a wheel cylinder piston causes each piston to push against each brake shoe with 2000# pounds of force.
    I don't know the technical term but I call it "hydraulic force multiplication". basically it is the reduction gear principle applied through hydraulics and is how a torque converter multiplies torque.
    This is why a manual master cylinder needs a smaller piston and more travel than one in a power assisted unit.
    I wont be using this formula but for research...the area of a piston is equal to pi(3.14...) times radius squared or A=(pi)R^2 or A= 3.142(R*R)
    The area of an engines piston is calculated the same way and the volume of the cylinder is the area times the stroke. Take that times the number of cylinders for your total engine displacement.

    back to the brake parts
    I used the stock 71 manual brake master cylinder and footfeed. The 1981 4-wheel-disc proportioning valve that came out of the car that I got the axle from, stock 1971 front rotors and calipers and stock 1981 rear rotors and calipers. The 2nd gen Fbody is supposed to have the residual pressure valves built into the proportioning valve.

    According to stainless steel brakes [URL="http://www.ssbrakes.com URL]
    Discs need a 2# backpressure and drums need a 10# backpressure.

    The manual footfeed has the pushrod mounted in the hole closest to the pedal pivot for greater leverage.

    The residual pressure valve could be in the proportioning valve or in the master cylinder of older cars with drums. This function has something to do with keeping the drums or wheel cylinder cups from retracting for firmer pedal feel. Too much will cause your pads to drag the rotor on a disc setup.




     
    Last edited: Jan 23, 2010
  2. RacerRick
    Joined: May 16, 2005
    Posts: 2,753

    RacerRick
    Member

    I would use the rear disks off of a fourth gen camaro with the intregal brake drum. Easier to find and the fab work is pretty much the same. They also don't have the parking brake issues, or seizing up problems of the 2nd gen F-body brake setup.

    I have that setup on the back of my 80 Z28. Stops good but lots of maintainance and very expense calipers. You also need to use the e-brake every time you park it to keep the caliper adjusters from seizing since this is how the brakes adjust.
     
  3. phat rat
    Joined: Mar 18, 2001
    Posts: 4,482

    phat rat
    Member

  4. bulletproof1
    Joined: Feb 23, 2004
    Posts: 2,080

    bulletproof1
    Member
    from tulsa okla

    i wouldnt use the caddy calipers ...they are very big and ugly and the park brake is a mess...
     

  5. torchmann
    Joined: Feb 26, 2009
    Posts: 787

    torchmann
    BANNED
    from Omaha, Ne

    They actually look just like the fronts. the early caddys do.
    I had a set of rear discs off a newer caddy I was going to use but noticed the caliper was quite a bit smaller and the rotor was a bit smaller than what's on the 77-81 transam. It still would have been a good swap but I just opted for the calipers that had more piston and pad area because they matched the fronts better and my car is heavy. about 3400 lbs. The parking brakes are a cinch with the right cables. Form follows function is my style

    I think the ones I like the best would be the willwood with the internal drum parking brake. The Willwood calipers and pads are universaly topps and I think they also have larger rotors than stock.
     

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