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Discussion Starter · #1 ·
I am having serious frustrations getting the brakes to work in my car, it has taken so long to get parts and everytime it is just a little better, but inadequate.

Recently I installed a vacuum canister and a 1" master cylinder from a 93 cobra for disc/disc. (real nice aluminum with a plastic resevoir)

Unfortunately the brakes will still not lock up and the braking in general is getting better but still sub par.

The system is now a 9" power booster, 1" MC cylinder, stock front calipers, SSBC rear disc brake conversion, rear slotted rotors, rear brake proportioning valve, and new lines and fittings.

I took a look at the mounting pivot of the brake pedal and it looks like it is a manual brake pedal linkage, please see the picture. From what I have read it looks to be a manual pedal, as the pivot is attached to the same place as the clutch pedal and there is an additional hole way up there for where a power pedal might mount.

Would this be the culprit in causing my braking troubles?
Using a manual pedal with a power setup?

Picture

http://myspace-011.vo.llnwd.net/01364/11/03/1364673011_l.jpg


<font size=-1>[ This Message was edited by: kleetuz on 11/1/06 6:42am ]</font>
 

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I don't think so - usually the difference between a manual pedal and a power brake pedal is the leverage applied. With a manual pedal you should get more leverage pushing against the master cylinder than with a power brake pedal. So you should be able to lock up the brakes easier with a manual pedal. What kind of cam do you have? Usually if you have a long duration cam (lopey idle) you won't develop enough engine vacuum to power the power booster. Get a gauge and measure the manifold pressure. You realistically need over 15"Hg to make the brake booster effective, however most places say 18" Hg.
 

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Wow, couldn't have said it better myself.
Your engine vacuum is the remaining variable.
Knowing that number is the key.
 

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Discussion Starter · #4 ·
I've measured the engine vacuum at idle, it is about 13-14", but as I mentioned I also just installed a vacuum canister to capture additional vacuum for the booster, but it didn't make much difference in stopping the car.
 

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Ckeck your shoes/pads for glazing...

What car are we talking about stopping here?

You should be able to lock them up fairly easily...

The pivot for a manual brake setup is slightly more advantageous than for power brakes because you need less external force for power brakes as was stated earlier.

There is something withthe shoes or pads or maybe the M/C you are useing...


FE
 

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Discussion Starter · #6 ·
Just a sub 3000lb 70 Mustang. I've tried several master cylinders with no luck. The pads are new and the rotors were resurfaced and I spent some time to bed them in.
I guess it will just take more time, to look into the pads, rotors, perhaps the calipers, though they seem to be operating normally, at least when the car is sitting still.
 

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Proportional valve?
 

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Read something in hot rod's tech section, guy with a gm something or other street rod, 50's car I think? anyways, he went to a big booster, and what hot rod tech guy suggested a smaller sized piston for his master cylinder...line pressure was not enough with the bigger booster after he swapped from manual to power brakes.
I swapped from a single diaphram unit on my 1972 Torino to a lincoln dual unit, but I had to modify my brake pedal pivot point as the setup I have originally had manual brakes. I compared my original 4 speed brake pedal to that of a automatic torino pedal and discovered a difference of over 2" in the pivot placement on the pedal.
I added the auto pivot onto my 4 speed brake pedal, and along with my conversion to 12.5" CV police rotors, and an-3 brake lines, it makes a hell of an improvement. I am still running rear drums however.
 

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13-14" of vacuum is definitely acceptable for a single
diaphram booster. Check out your booster. With the
engine off, press the brake pedal several times. This
will bleed off the booster's vacuum assist. Pressing
lightly on the brake pedal, start the engine. Pedal
should drop slightly under your foot if your booster
is operating correctly. If you have no drop in pedal,
you have booster issues. As you can probably guess,
no booster assist means pedal effort is much higher
than normal.
I'd also get some pressure gauges, take out the bleeder
fittings and install them. The psi needs to be significant
at each corner of the car. Without having Schwarzenegger
pushing the brake pedal, I'm thinking maybe 20# at the
pedal better be giving you about 1000 psi at the caliper.
 

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Discussion Starter · #11 ·
Some data on the vacuum situation for my brakes. Testing the brake booster by turning the car on with my foot on the pedal says that it is working, but here's what I found out using a vacuum gauge.

Measured the vacuum today at various locations and while driving around using a better gauge than I have used before.

12" measured at intake manifold at idle.

Gauge connected to vacuum canister port for booster, canister check valve connected to manifold shows 12" at idle, after a few revs will get up to 20" and slowly decrease to 12", takes about 20 seconds. -Questionable since the vacuum canister check valve should prevent vacuum from dropping within the canister right?

Then I put a vacuum T off of the brake booster, with the canister connected to the brake booster and gauge off of the T at the brake booster. While driving around vacuum only gets up to 18" max and quickly decreases to 12", even when I'm not using the brake. Since I had the gauge visible while driving I was able to see that when braking the brakes are grabbing decently at first, while vacuum is between 14"-18", but it drops rapidly and the car rolls to a stop, even though I am standing on the pedal quite hard, and no lock up will occur.

Rotors and pads appear to be good, no glazing. My thoughts are that it seems like the brake booster is not holding vacuum, which sucks because it is a brand new booster unit.

Any input?

Additionally I think I will get the SSBC pressure gauge kit and measure the pressure in the line since it is not too expensive, didn't know they offered that, thanks.
 

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Get the vacuum pump off of an 86-88 Cadillac De-Ville 4.1L at a wrecking yard. They are 12V and are located under the left front fender.
Put this in your car and run it to your brake booster only. DIsconnect the rake booster from your engine.

Test your stopping power with it now!

OR.

open all 4 bleeder screws.

Get in the car and slam the brake petal to the floor holding it there until the 4 bleeders are closed.

Check which bleeders shot the furthest/farthest, and post your results.

You could have weak delivery to the rear or to the front...

FE
 
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