Brake line pressure

Shocks, Springs, Brakes, Frame, Body Work, etc

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BirdMan
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Re: Brake line pressure

Post by BirdMan » Sun Jul 08, 2018 12:58 pm

On my 65 Falcon V-8 car I replaced the front 10" drum with Wilwood 4 piston disc and have Ford 11"x 2"?? with 1-1/16" wheel cylinders and no proportioning valve. I figured if I needed one I would install it then. I replaced the original mc with a 15/16" piston dual mc. First time down the drag strip I applied same pedal pressure as I normally would and it stopped so quickly I could have taken the short/first return cutout :) so I let off and coasted to normal return cutout and gently applied enough pressure to turn.

True, the car was a manual mc to begin with and didn't have to mess with pedal ratio, thankfully!

I am just posting my experiences. I don't remember pressure needed. I usually used 300 lbs on linelocked rear wheels to stage from prestage with my stick car, enough to keep from rolling when I revved to 8,000 or better.
Dale C.

aquartlow
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Re: Brake line pressure

Post by aquartlow » Sun Jul 08, 2018 3:27 pm

Since you are wanting a bit more brake actuation to rear drums, might want to try one size smaller bore wheel cylinder. I did this by mistake on an older Cougar and the rear brake effect was more aggressive, just by going from 1" to 15/16" WC bore. Hope this helps.

chimpvalet
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Re: Brake line pressure

Post by chimpvalet » Mon Jul 09, 2018 6:29 pm

Sorry, I disagree on that. A reduced diameter w/c is a reduced area w/c and that reduces the force applied to the shoes, all other pieces remaining as before. Simple calculation: line pressure versus area of cylinder to which it is applied equals force from the cylinder (slave end). So 300psi applied to a 1" cylinder (area 0.79" sq.) gives 237 lb. force, whereas 300psi applied to 15/16 w/c (area 0.69" sq.) gives 207 lb.

chimpvalet
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Re: Brake line pressure

Post by chimpvalet » Mon Jul 09, 2018 6:30 pm

. . . unless you meant reduce the FRONT w/c.

aquartlow
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Re: Brake line pressure

Post by aquartlow » Tue Jul 10, 2018 6:29 pm

chimpvalet wrote:
Mon Jul 09, 2018 6:30 pm
. . . unless you meant reduce the FRONT w/c.
I was speaking of the rear drums and you are correct about increasing bore size to increase braking effect. I had my wires-crossed, fingers were typing faster than brain could keep up, sorry for the mis-information.

tjz450
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Re: Brake line pressure

Post by tjz450 » Fri Jul 13, 2018 11:45 am

I went thru this same issue back in the day with a 68 Camaro. Similar size car and similar parts as your friend's 65 Chevelle I think. I tried every master cylinder for Chevy's I could get, including a 1 1/4" truck master. I didn't want power assist, and I never had trouble locking the brakes up. But in my opinion, 13/16" master is too small. Stock is 7/8" or 1" with drums. I liked the brake pedal high and tight. Smaller masters had too much travel for my comfort. Maybe your friend should have done a power brake booster. Most all chevys with front disc had power assist. I wound up with a 1 1/16" Chrysler master and wilwood 10 1/2" disc kit on the front.

Calypso
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Re: Brake line pressure

Post by Calypso » Sun Jul 15, 2018 4:21 am

Pad material not mentioned yet. Some pads need to be up to temp before they begin to work well.
FWIW, I have very positive experience from Porterfield R4-S Pad on the street.

mk e
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Re: Brake line pressure

Post by mk e » Mon Jul 16, 2018 8:36 am

I had a 65 vette with manual brakes. The first time I drove it I expected to need to push hard on the pedal and rearky put myself through the windshield...that car stopped!

I don't recall noticing a lot of petal travel, but it was 4 wheel disc. Replacind the rubber hoses with braided SS lines cut pedal travel to lock in about 1/2, or at least that's the feel, I never actually measured it, but the feel is completely different, pedal feel hard as rock when you lean on it.
Mark
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