Brake help
#2230896
01/09/17 04:49 PM
01/09/17 04:49 PM
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Joined: Jan 2017
Posts: 35 San Antonio.TX
JohnGlenn
OP
member
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OP
member
Joined: Jan 2017
Posts: 35
San Antonio.TX
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I'm stumped, need some help. Background first. 73 Challenger, stock disk drum power brakes except wilwood proportioning valve when factory unit died. Moved from Abilene TX to San Antonio, once in San Antonio brakes would get super hot in traffic and eventually pedal would bottom out, I assumed crappy old brakes, planned an upgrade and never tried to fix, just stayed away from traffic.
Upgraded front brakes, torched disk off old rotor and turned remainder to make a hub, then used Baer 13 inch rotors and calipers off a sn95 cobra mustang purchased used for cheap. Machined bracket from 6061 aluminum, lines from Cass at Dr. Diff.
Bled crap out of brakes, assumed old fluid crap from boiling, so bled/flushed with fresh dot 4. Pedal is rock hard without booster assist, fluid is clear with no bubbles. Test drive yielded unimpressive results, granny brakes at best. Felt like pedal was bottoming out before enough pressure achieved.
Tested a ton, here's what I have. With assist, apply brakes and you can feel it hit a stop, feels mechanical, even sounds metallic. Applying pressure when on this stop I tried bleeders. Open rear, no effect, open front and pedal falls slowly all the way to the floor even with rear bleeders closed.
Neighbor is ASE Master, he thought master was bad, $25 later no change. Cass at Dr Diff thought plunger adjustment, possibly bad proportioning valve; checked plunger with mic it's good, took prop valve out and replaced with hard line, no change.
It's like linkage is binding, or master is bottoming out, but as soon as you open a front bleeder the pedal falls. So, I'm not out of volume, and I don't have enough pressure to mock anything up. Idea?
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Re: Brake help
[Re: Andrewh]
#2231044
01/09/17 08:06 PM
01/09/17 08:06 PM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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On a toughie I plug the MC ports with a pair of brass inverted flare male plugs from the Edelman fittings cabinet at your parts house. If the booster, round nub pushrod clearance, MC piston seals, MC itself bled out, all of these are OK then at this point with it capped, the pedal will be rock hard with virtually no travel (eng idling). try light pressure and try a heavy stomp. that'd be how I would start & the fittings are cheap & you will reuse em in your career. we gotta pin this down a bit.
live every 24 hour block of time like it's your last day on earth
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Re: Brake help
[Re: JohnGlenn]
#2231286
01/10/17 12:58 AM
01/10/17 12:58 AM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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take a vacation for several days then pickup the brass plugs
live every 24 hour block of time like it's your last day on earth
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Re: Brake help
[Re: moparx]
#2231774
01/10/17 06:34 PM
01/10/17 06:34 PM
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Joined: Nov 2003
Posts: 36,041 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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^this^ I just came back from my parts house & the current Edelman screen does not show the brass "plugs" that they used to carry, the 1/2 and the 9/16 ones that I have priorly purchased.
live every 24 hour block of time like it's your last day on earth
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Re: Brake help
[Re: JohnGlenn]
#2231903
01/10/17 10:31 PM
01/10/17 10:31 PM
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Joined: Jan 2007
Posts: 7,664 IN
ahy
master
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master
Joined: Jan 2007
Posts: 7,664
IN
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Interesting, how did your pedal feel before the master booster change? I'm running 13 inch rotors also. With the Cobra calipers and OE style booster/master the pedal felt OK in moderate braking. If I pushed harder it was like hitting a hard mechanical stop. I think I was just running out of assist. With the PB linkage ratio, bigger master and smaller calipers it just does not work well at all without assist. Hydroboost is an option or the smaller master + stronger booster I used. In my experience, one or the other is needed.
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