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Brake help #2230896
01/09/17 04:49 PM
01/09/17 04:49 PM
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San Antonio.TX
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JohnGlenn Offline OP
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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?

Re: Brake help [Re: JohnGlenn] #2230990
01/09/17 06:56 PM
01/09/17 06:56 PM
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Andrewh Offline
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so a couple of items.
I am shocked dr diff couldn't help, but let go through this again.

you have a 73 challenger that came factory with power drum brakes.
you swaped the drums for baer discs up front.
you do not have a factory proportioning valve, but a wilwood adjustible.
you have replaced the master cyclinder.

questions
did you replace the master with a factory drum master? or the disc?

do you know the size of the piston in the mc?

did you mess with the lines to the master during the swap?
rear port to front brakes and front port to rear?

did you replace the soft lines/hoses for the front and rear?

did you mess with the adjustible proprtioning valve once you changed to disc? as it sounds like it was on before your swap.

Re: Brake help [Re: Andrewh] #2231044
01/09/17 08:06 PM
01/09/17 08:06 PM
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Lincoln Nebraska
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RapidRobert Offline
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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
Re: Brake help [Re: JohnGlenn] #2231060
01/09/17 08:27 PM
01/09/17 08:27 PM
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San Antonio.TX
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JohnGlenn Offline OP
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Car was originally disc/drum, master is for disk drum, 1 1/32 bore.

Prop valve was/is completely removed right now, still can't look up either end.

Lines are right, closest to booster to front, farthest to rear. Front lines replaced with stainless braided, rear rubber line is about 7 years old.

Rapid Robert - I'll try to get those plugs, great idea to isolate the problem.

Re: Brake help [Re: JohnGlenn] #2231079
01/09/17 08:53 PM
01/09/17 08:53 PM
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Irving, TX
feets Offline
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Welcome aboard!

You changed the calipers. Did you measure them to see if the master cylinder was the proper size for them?


https://board.moparts.org/ubbthreads/ubbt...ed-to-know.html


We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
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Re: Brake help [Re: JohnGlenn] #2231105
01/09/17 09:31 PM
01/09/17 09:31 PM
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San Antonio.TX
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JohnGlenn Offline OP
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I did not measure them, what I installed is essentially Dr Diffs stage 4 kit. Cass said any factory master should push them, he advised 7/8 for manual, but 1 1/32 is fine with power brakes.

Re: Brake help [Re: JohnGlenn] #2231171
01/09/17 10:56 PM
01/09/17 10:56 PM
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ahy Offline
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I run a '70 Challenger with later disc spindles... probably similar to your setup. Also upgraded discs with Cobra calipers. I run 13" rotors front... I don't think you said what size you run.

Anyhow, the Cobra calipers have significantly smaller piston area vs Mopar OE single piston discs. They need more line pressure. I started with a 9" OE single booster and OE spec disc/drum master. Brakes were OK but not 100%. Granny brakes? It took a Dr Diff 15/16" master and dual 8" booster to get to 100%.

I wonder if you just need more line pressure than your current setup can provide?

Re: Brake help [Re: JohnGlenn] #2231212
01/09/17 11:40 PM
01/09/17 11:40 PM
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San Antonio.TX
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JohnGlenn Offline OP
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Interesting, how did your pedal feel before the master booster change? I'm running 13 inch rotors also.

Re: Brake help [Re: JohnGlenn] #2231257
01/10/17 12:24 AM
01/10/17 12:24 AM
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San Antonio.TX
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JohnGlenn Offline OP
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Honestly, by the time I sort this out, rip my hair out and lose 10 years off my life. I'm thinking about just converting everything to 99-04 cobra gear. Rear disks, master, hydroboost and factory proportioning valve. Am I nuts?

Re: Brake help [Re: JohnGlenn] #2231286
01/10/17 12:58 AM
01/10/17 12:58 AM
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Lincoln Nebraska
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RapidRobert Offline
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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
Re: Brake help [Re: JohnGlenn] #2231448
01/10/17 08:18 AM
01/10/17 08:18 AM
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Georgia
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magiccuda Offline
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Can someone post the part numbers for pipe plugs. I have looked for them until im sick. No one around hear has them they dont stock plugs in these sizes, maybe I could order them. Only thing I could find are npt thread, I needed 9/16 x 20 and 1/2 x 20.

Re: Brake help [Re: magiccuda] #2231597
01/10/17 01:56 PM
01/10/17 01:56 PM
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north of coder
moparx Offline
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i have a couple of "weatherhead" fitting part numbers that adapt the master threads to standard 3/16" brake line. they are : WHD 7909 and WHD 7912. you could then use standard 3/16" line plugs to cap off the master ports. or, you could find a junk master cylinder from a car that has the lines still connected, chop off a short section of the line next to the master fittings, then braze the line and fitting together, thus creating a plug. i have done this many times when the correct plug wasn't available, or the parts store was closed. just something to consider.
beer

Re: Brake help [Re: moparx] #2231774
01/10/17 06:34 PM
01/10/17 06:34 PM
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Lincoln Nebraska
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RapidRobert Offline
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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
Re: Brake help [Re: JohnGlenn] #2231903
01/10/17 10:31 PM
01/10/17 10:31 PM
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ahy Offline
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Originally Posted By JohnGlenn
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.

Re: Brake help [Re: JohnGlenn] #2232015
01/11/17 12:39 AM
01/11/17 12:39 AM
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Georgia
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magiccuda Offline
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moparx you got the answer thx

Re: Brake help [Re: magiccuda] #2232208
01/11/17 11:55 AM
01/11/17 11:55 AM
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north of coder
moparx Offline
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up let us know how you make out.
beer

Re: Brake help [Re: ahy] #2232465
01/11/17 08:39 PM
01/11/17 08:39 PM
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San Antonio.TX
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JohnGlenn Offline OP
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San Antonio.TX
This sounds like what I'm dealing with. And would make sense, I'm really borderline on vacuum, and with planned changes it's only going to get worse. I'm going to have to braze plugs, San Antonio just doesn't have a real parts house, but if that shows good I think I'll just go hydroboost and upgrade the rears while I'm at it.

Re: Brake help [Re: JohnGlenn] #2232475
01/11/17 08:56 PM
01/11/17 08:56 PM
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Morristown Tn.
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71birdJ68 Offline
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I thought you died last month?

Re: Brake help [Re: 71birdJ68] #2232519
01/11/17 10:14 PM
01/11/17 10:14 PM
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San Antonio.TX
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JohnGlenn Offline OP
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Originally Posted By 71birdJ68
I thought you died last month?



Shhh, don't stare a the conspiracy... 😎

Re: Brake help [Re: JohnGlenn] #2233445
01/13/17 12:35 PM
01/13/17 12:35 PM
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Posts: 35
San Antonio.TX
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JohnGlenn Offline OP
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So I made plugs and tested the master. Nice solid pedal, all the way on top. I think ahy is right, it just not enough pressure for the smaller pucks. I think everything is performing as advertised, that's why it's so hard to nail down, nothing is failing. I believe it's a combination of large master not making enough pressure, coupled with an under performing booster because she only pulls about 12 inches of vacuum.

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