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I am trying to diagnose issues with a retrofit disc/power brake system on a 66 Ford Mustang. After the conversion, there was a spongy pedal feel, but there was no air in the lines.

We bench-bled the master cylinder and capped the output ports to test it. Now, when applying pressure to the master cylinder piston, On the bench, with the ports plugged, i can push it in maybe 1/4" - 1/2" until it feels "solid". Is this normal? Should there be this amount of play?

  • Is the 1/4-1/2" movement with it in the vehicle and hooked up or still on the bench with the ports plugged? – Pᴀᴜʟsᴛᴇʀ2 Jun 13 '17 at 0:00
  • Paulster2 this is still on the bench with the ports plugged. – PhilippNagel Jun 13 '17 at 11:14
  • I don't know for sure with your specific unit, but that seems like the movement is pretty big. Think of the arc travel as it's translated to the brake pedal. A 1/2" probably will translate to 2-3" at the pedal and that's before you factor in the cush for the rubber lines at the wheels. – Pᴀᴜʟsᴛᴇʀ2 Jun 13 '17 at 18:48
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Yes, this is normal, if you ever rebuilt a MC you will see there are springs behind the cups on the pistons, when installed in the car and properly bled you will get about half the distance or a little more to the floor in pedal travel for power brakes, about 1/3 or less with manual brakes.

Additional note:

For retrofit disc brakes, when mounting the MC to the booster be sure there is proper clearance (0-1/16") between the booster pin and the MC piston, most Boosters have an adjustment screw on the pin, some are adjusted properly and some are not, when mounting the MC to the booster there should be no resistance before the MC is flush with the booster, do this check by hand(do not use the mounting nuts), if the MC does not fit flush with booster with No resistance, then you have to screw the booster pin in until there is clearance between it and the MC piston.

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Check to determine the presence of a master cylinder spring/rubber plug to cushion/isolate the pedal from the “hard” hydraulic system. Since this is a retrofit I’m WAGGING here.

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You said this was a conversion, so Im just making the assuming that the bore and stroke of these replacement parts may not match the original parts, thus the MC volume, to brake piston total volume, may not match the ratio that once worked well...

In lieu of posting future dead links here, Ive read that the golden ratio, brake to MC, should be between 23 to 27, below and the brakes may feel real stiff, and above they may be spongy or never substantially close.

Not to say disc size and wheel size dont have anything to do with the feel...

I've been looking at this hard as in the middle of diagnosing a bike that came spongy from the manufacture, low production low QC, and well may have been produced with a poor ratio...

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