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Old 11-12-2011, 08:01 AM
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Brakes, let me run this by you...

Hi All,

The brakes on my Maverick royally s^ck! I have an MII front conversion clip from RCMotorsports using their 11" discs up front and a 9" out back with Ford Motorsport 11x2 drums. The master cylinder is a new 15/16" bore from a '75 Granada with manual disc/drums, along with the combi valve. The pedal is high and (very) hard, but the car seems to take forever to stop. Only two things come to mind, the extra large rear drums and the fact that the lines from the M/C to the combi valve loop up a bit higher than the fluid level. But wouldn't that make the pedal mushy ? ? ?

A friend with a very similar car (but smaller 10x1.75 drums) is running his without any proportioning or combi valve at all, just straight from the M/C to the wheel cylinders. His thinking is that the larger rear tires are the "bias" and a combi is not needed. He's autocrossed a few times and street drives it regularly without any braking problems at all, in fact swears by it. Says it stops like any "modern" car and the pedal is not a leg breaker.

Any (open-minded) thoughts on this? I've got to do something in order to feel safe about driving mine...

TIA!

Russ

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Old 11-12-2011, 07:55 PM
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Hey S10. I'm no expert on brakes but being that this is a custom set up in your car and that your friend has almost the same setup except for the Prop valve, i would see if eliminating it would solve your problem. Hopefully others will chime in
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Old 11-13-2011, 08:36 AM
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sounds good to me. you may want to try a larger master cylinder. something people overlook is pedal to leverage ratio. i dont remember the particulars, but measure from the pedal pivot to the push rod and the foot pad to pivot on your car and your friends car. if theres a difference,you may need to change it. push rod closer to the pivot will give you better leverage on the pedal, farther from the pivot will decrease your leverage and increase the effort needed by you to push the pedal.
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Old 11-13-2011, 09:35 AM
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Quote:
Originally Posted by seniormaster
sounds good to me. you may want to try a larger master cylinder. something people overlook is pedal to leverage ratio. i dont remember the particulars, but measure from the pedal pivot to the push rod and the foot pad to pivot on your car and your friends car. if theres a difference,you may need to change it. push rod closer to the pivot will give you better leverage on the pedal, farther from the pivot will decrease your leverage and increase the effort needed by you to push the pedal.
I'm already there. The smaller the M/C diameter, the easier the pedal and 15/16" is about the smallest you can go and still move enough fluid to actuate the front brakes. IIRC, the older 4-wheel disc Corvettes had a 1" bore. My pedal ratio is 6.5:1 which is the most one can get w/o modding things...

Keep 'em coming!

Russ
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Old 11-13-2011, 12:04 PM
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look here for what you need.This guy is da bomb
http://www.hotrodsusa.com/store/tech.html
Dean puts it all into perspective, lots of good info there to read.
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Old 11-14-2011, 08:57 AM
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Quote:
Originally Posted by latech
look here for what you need.This guy is da bomb
http://www.hotrodsusa.com/store/tech.html
Dean puts it all into perspective, lots of good info there to read.
Thanks for that! I'd actually read that article before, but didn't bookmark it and lost track of it... He brought up one concern I hadn't thought of, using a m/c with too small a bore may not move enough fluid to fully actuate the front brakes. After googling around a bit, I found a bunch of threads on G-body (I have those calipers in my kit) manual brake conversions that were using an even smaller (7/8") bore cylinder than I have, so I'm OK there. He did say that the factory combi-valve should only be used for a scenario that mimics that application, otherwise toss it! Next, I'll be looking for some new parts to finish 'er up.

BTW, I found out that my m/c, even though designed for disc/drum, has no residual valve built into the rear circuit as most do. That and the fact that it's a "smooth bore" without the larger "quick take up" step bore may be causing some issues.

Thanks!

Russ
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Old 11-15-2011, 03:12 PM
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'Nother question, because I'm seeing conflicting information concerning this:

Do I need residual pressure valves, front and rear? I've seen where you only need them 'if' the m/c is lower than the wheel cylinders, and I've also seen where they're recommended for all uses...

Russ
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Old 11-15-2011, 05:06 PM
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I could be wrong , but IIRC , 70's granada, monarch, pinto, MII,bobcat, used virtually all the same brake parts. Have you investigated the internals of your prop. valve ? When I was building my car I think I went thru 3 or 4 before I found 1 that would clean up enough to be servicable , lots of crap gets deposited in those valves , also , the way I understand it , w/ your setup , you don't need or want residual valves.


dave
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Old 11-15-2011, 05:41 PM
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Quote:
Originally Posted by 2old2fast
I could be wrong , but IIRC , 70's granada, monarch, pinto, MII,bobcat, used virtually all the same brake parts. Have you investigated the internals of your prop. valve ? When I was building my car I think I went thru 3 or 4 before I found 1 that would clean up enough to be servicable , lots of crap gets deposited in those valves , also , the way I understand it , w/ your setup , you don't need or want residual valves.


dave
Dave,

I disassembled everything except the m/c (which was new), cleaned and reassembled them. I'm confident the factory combi-valve is at least as clean as new and fully operational.

My m/c is the same as mentioned in this post , which does not have a resid valve as per the guy who sells them in his brake kits. I will certainly check whenever I remove the lines for my "redo".

After reading the article (link that latech posted), I'm gonna do away with my combi *because* my ride is obviously much different than what the valve was intended for. Also a good section on the resid valve:

Quote:
Originally Posted by from the Dean Oshiro article:
Residual Valves: Residual valves are pressure valve use to retain pressure in the lines. The most common use is on a hotrod when there is a floor mounted brake pedal and master cylinder. Mounting the master cylinder (M/C) below the floor positions it below the calipers. Gravity will cause the fluid to flow away from the calipers. The residual valve will retain pressure within the lines. (i.e. 2 pounds residual valve will retain 2 pounds of pressure, 10 pound will retain 10 pounds.) Drum brake master cylinders have residual valve(s) built into the master cylinder. This is needed to maintain pressure against the cup seals in the wheel cylinders. If you are using a disc brake master cylinder or after market you will need to install a 10 pound residual valve for the drum brakes. Do not install a residual valve if your master cylinder already has one in it. This will cause the brakes to lock up after the second application to the brake pedal.
It appears that the resid valve prevents full retraction of the rear shoes and keeps the cylinder seals loaded. So, I ordered a pair of Wilwood resids from Speedway, along with the proper fittings to adapt them to my lines. Prolly won't get to it for awhile though...

Russ
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