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
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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: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
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: 36,040 Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
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Posts: 36,040
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: 36,040 Lincoln Nebraska
RapidRobert
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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
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master
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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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Re: Brake help
[Re: JohnGlenn]
#2233468
01/13/17 01:29 PM
01/13/17 01:29 PM
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Joined: Jan 2003
Posts: 28,067 Irving, TX
feets
Senior Management
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Senior Management
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Irving, TX
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Did you look at the brake math spreadsheet and find the optimum master cylinder size for those calipers?
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind. - Stu Harmon
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Re: Brake help
[Re: JohnGlenn]
#2234091
01/14/17 05:32 PM
01/14/17 05:32 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
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Circle Track
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Lincoln Nebraska
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I could see a less than perfect size booster/MC causing a problem but from the descrip it sounds like there is a mechanical failure/issue somewhere. A suggestion since the booster (even if undersized) & the MC are good/bled is rehook the MC line to the front discs next to the firewall port only & bleed it out & work with them first & see if I can get it to straighten up
live every 24 hour block of time like it's your last day on earth
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Re: Brake help
[Re: JohnGlenn]
#2234449
01/15/17 02:36 AM
01/15/17 02:36 AM
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Joined: Jan 2017
Posts: 35 San Antonio.TX
JohnGlenn
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So, I kind of confirmed my suspicions tonight. I eased it over to an empty parking lot right outside my neighborhood, instead of being nice I figured the best vacuum would be achieved by jumping all over it. So, I lit them up from a 20mph roll, backed off and jumped on the brakes...front end dropped and locked both front wheels up, might have locked rear also but I'm unsure. So it's settled, lack of vacuum.
Last edited by JohnGlenn; 01/15/17 02:38 AM.
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Re: Brake help
[Re: JohnGlenn]
#2234458
01/15/17 02:51 AM
01/15/17 02:51 AM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
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Alright, would that be the eng not generating enough vac (wild cam) or wrong booster?
live every 24 hour block of time like it's your last day on earth
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Re: Brake help
[Re: JohnGlenn]
#2234485
01/15/17 03:51 AM
01/15/17 03:51 AM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
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I ain't into PB but iirc someone (on here) said that either 12 or 14 in hg is needed for good functioning. If that ain't available, is there a booster type that will function with less? Or maybe a vac pump.
live every 24 hour block of time like it's your last day on earth
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Re: Brake help
[Re: Sxrxrnr]
#2234600
01/15/17 01:11 PM
01/15/17 01:11 PM
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Joined: Nov 2003
Posts: 36,040 Lincoln Nebraska
RapidRobert
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Circle Track
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I would myself prefer to go non-power brake in that case. that'd be my choice (even with adequate vac): less weight/better brake feel/simpler system/more room. EDIT From what I understand PB does not give increased braking, it just requires less leg effort for the input side.
Last edited by RapidRobert; 01/15/17 01:12 PM.
live every 24 hour block of time like it's your last day on earth
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