Brake help

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?

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.

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.

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.

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

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.

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?

Interesting, how did your pedal feel before the master booster change? I'm running 13 inch rotors also.

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?

take a vacation for several days then pickup the brass plugs

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.

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.

^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.

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.

moparx you got the answer thx

let us know how you make out.

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.

I thought you died last month?

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

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.

Did you look at the brake math spreadsheet and find the optimum master cylinder size for those calipers?

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

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.

Alright, would that be the eng not generating enough vac (wild cam) or wrong booster?

I'm banking on engine. It's a 318 punched to a 393, hughes HYD roller 236/242 duration with 568/560 lift, 11:1 compression, worked iron 2.02/1.60s, matched air gap topped off with an 850 Holley. So, not real good vacuum.

And it's only going to get worse. I've got a set of fully worked w5s and a matched single plane just waiting to go on, so, my vacuum predicament isn't going to get better.

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.

Hydro boost, albeit a bit too non period correct looking for me. That is if low vacuum should prove to be the problem. 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.

The car isn't factory correct, and I don't mind. If it were a 70-71, I would care, but being a 73 it's subject to my whims. What I want is a solidly streetable car, something you drive like a new car, something that's not laborious to drive. Most of you know what im talking about. I want to Friday night cruise without the pucker factor. I, like many car guys, love my ride, but I don't trust it when traffic gets hairy. I've already started down the pro touring line, now it's time to commit.