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Old 09-06-2014, 02:44 PM
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Help with brakes!

I just can't get a decent pedal and I don't know why. I have a TCI chassis under a '40 Ford coupe. It came with 11" front discs with the typical 2.38" bore S10 calipers and 11" x 2.25" rear drums. Also has a 7" dual diaphragm booster with a Corvette style 1" master. Since the master is lower than the brakes it had the required 2lb front and 10lb rear inline residual valves, and an adjustable prop valve in the rear line.*

I could never get what I would call a high pedal, and under hard braking wondered if I was going to run out of pedal travel before running out of brakes. I'm aware of the issues with bleeding a master under the floor and have used a power bleeder and also gone to the trouble of jacking each end of the car way up to get the low side brakes lower than the master while bleeding. *I'm fairly confident I got all the air out of the system.*

I've also replaced the master - three times.*

I've adjusted the pushrod between the pedal and booster. There's only about 1/8" of travel from being adjusted all the way out, to the point where the brakes start locking up after driving for a few miles (I believe the fluid is heating up and applying the brakes because there's no free play in the linkage).*

I thought I'd just have to live with this seeing as it appears that the way the linkage is designed means a lot more brake travel is required than normal to push the pushrod into the booster due to the cantilever design.*

So, I thought I'd try another approach - increase braking force for given pedal travel. I upgraded to 2.75" calipers up front, and a rear disc setup in back (11-1/8" rotors with 2 - 1/8" calipers). I changed my rear 10lb residual valve to a 2lb. I also went up to a 1-1/8" master cylinder. All else being equal the master should have raised the pedal and required more effort to press. *This is not the case. I think it's even lower than before.*

I investigated installing a 8" dual diaphragm booster, but there's just no way no how it's going to fit in the frame.*

I figured I'd ask if anybody has any tips, seeing as a good number of cars here have the same style brakes.

Help!

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Old 09-06-2014, 03:11 PM
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I'm no brake expert, but I think it's likely that your master cylinder simply can't supply the required volume of fluid to activate your brakes and have a decent pedal.
I had a similar issue with my 48 Plymouth. Front discs (S10), 10" rear drums, and a manual master cylinder (1" bore), and hanging pedal assembly from a 76 Valiant with disc brakes. Had brakes, but the pedal was really low, scary low. Replaced the master cylinder with one intended for a mid 80s Dodge pick-up (1 1/8" bore). Had a decent pedal, but I really had to mash on the pedal to stop. I don't think I could have locked them up despite my excess weight. I extended the brake pedal arm by about 2" to give more mechanical advantage, and the brakes work great now. Another thing that you probably already checked, but make certain your drum brakes are properly adjusted.
Good luck. Stopping is right up there on the priority list!!

Wayne
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Old 09-06-2014, 06:16 PM
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There is another issue that may be affecting your brake performance. I forgot to mention it in my earlier post. It seems that some (maybe all??) of the GM metric calipers were of a "low drag design", in that the piston retracted some distance INTO the caliper, so that there was no chance of drag against the rotor. That meant that additional brake fluid is required to push the piston back OUT to force the pad against the rotor. There's probably not much distance involved in how far the piston retracts, but even a small amount will be felt when you step on the brake. I've been told, but was unable to verify it, that SOME of the GM calipers were not of the "low drag" design. But all that's available are rebuilt units, so who knows what you're getting? Hope this helps you figure out your dilemma!

Wayne
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Old 09-06-2014, 10:49 PM
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What's your pedal ratio?
You want about 4.5 to 1.
Oldguy fixed his by adding length to his pedal and getting his ratio in line.
Check yours.
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Old 09-07-2014, 03:07 AM
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Quote:
Originally Posted by Oldguy48 View Post
There is another issue that may be affecting your brake performance. I forgot to mention it in my earlier post. It seems that some (maybe all??) of the GM metric calipers were of a "low drag design", in that the piston retracted some distance INTO the caliper, so that there was no chance of drag against the rotor. That meant that additional brake fluid is required to push the piston back OUT to force the pad against the rotor. There's probably not much distance involved in how far the piston retracts, but even a small amount will be felt when you step on the brake. I've been told, but was unable to verify it, that SOME of the GM calipers were not of the "low drag" design. But all that's available are rebuilt units, so who knows what you're getting? Hope this helps you figure out your dilemma!

Wayne
If this is the case wouldn't I be able to "pump up" the pedal?
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Old 09-07-2014, 03:09 AM
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Quote:
Originally Posted by RWENUTS View Post
What's your pedal ratio?
You want about 4.5 to 1.
Oldguy fixed his by adding length to his pedal and getting his ratio in line.
Check yours.
Yeah. I have no idea how to determine that with this style brake pedal...

I already have a huge amount of travel in the pedal.
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Old 09-07-2014, 05:12 AM
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Originally Posted by CFster View Post
If this is the case wouldn't I be able to "pump up" the pedal?
Yes, to some extent, but if you get in situation where you need to hit the brakes NOW, you don't have time to "pump up the pedal".

Wayne
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Old 09-08-2014, 09:26 AM
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Most likely you have air trapped some where or the rear drums are maladjusted. Here how to diag....

Get a piece of brake line with the rear master cyl fitting that is about a 1/2 inch long and weld or crimp the end shut. Install it on the master in the rear circuit and see if you've got a pedal. If you do, then you may have air trapped in the rear circuit or they are mis-adjusted. If the pedal is still not hard there is air in the front circuit or some other problem in. Once you know you can focus. DISCLAIMER: Don't drive the car in this diagnostic position.

Did you bench bleed the master before installing it? Are the rear drums adjusted properly?

Also go back to the 2.38 caliper and 1 1/8 master.
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Old 09-08-2014, 10:09 AM
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It's pretty simple to determine ratio on the pedal. Just measure the length of the pivot point to the operating rod, and then the pivot point to the center of the pedal. Divide the first measurement into the 2nd, and you have your ratio.
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Old 04-07-2015, 09:20 PM
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I would like to see a picture of the rocker as installed? The new master is for 4 wheel disc?
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