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#34344 - 02/05/07 06:34 PM brake ratio?
Anonymous
Unregistered


My dad has 37 ford street rod,it has a brake pedal with 6 to 1 ratio,for his brakes to work with his new abs system he needs a 4 to 1 ratio,does any know how to change the ratio? Cant find anyplace that makes different pedal,the people at abs said cut the one he has,and make it a4 to 1, anyone have any experence with this?

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#34345 - 02/05/07 07:04 PM Re: brake ratio?
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
pedal ratio is detirmined by dividing the length of the arm [the pedal pivot to the pedal pad center] by the distance from the pivot to the master cylinder pushrod hole. the FARTHER away from the pivot point, the lower the ratio. you maybe able to fix this by just drilling a new master cylinder rod attaching hole in your pedal. the thing to watch for, however, is that you don't bind the rod in the master at full stroke. hope this helps.

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#34346 - 02/09/07 01:14 PM Re: brake ratio? [Re: moparx]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
OK, I need a clearifacition on this. I bought a brake pedal "kit" that included the brake pedal assembly, the dual 7" brake booster, and the "corvete" style master with the brake line fittings on both sides of the master. Its suppose to work on disc/drum brakes. I also bought a "GM" style poroprting valve. This is installed on my 39 Plymouth and I have a disc brake conversion with Volari rotors and chevy truck calipers. The rear brakes are 9" drums from a Dakota. It has all new lines (3/16") and all new hoses, the calipers were rebuilt (napa) and there are new rear wheel cylinders. The master was bench blead before being installed and gave good fluid supply from both fittings.

Pretty much, these brakes suck. The pedal pushes real hard (motor has 17" vacume at idle) and the rear brakes do not work. With the rear axle off the ground, at idle in gear (auto trans) you can have to push with both feet to stop the rear wheels from turning! The car stops with front brakes only. The brake pedel is firm and is not bottoming out, if you open either a front or a rear bleeder screw the brake pedal goes lower. There is no air in the system, and I can see the rear wheel cylinders move when the drums are off (least the driver side).

The pedal bracket is mounted to the firewall with 4 bolts and extends to the bottom of the dash where it is also bolted. The brake arm is 15" long from the pivot to the center of the brake pad and there is a master cylinder arm that is 5" long from the pivot to the center bolt for the master. There is about 1/4" to a 1/2" of deflection in the firewall when you push the brake pedal (need to add some bracing).

Things I have done that have made no difference: put washers between the master and the brake booster thinking the master wasn't returning all the way. Just got a lower pedal. Adjusted the bolt between the booster and the brake pedal-both directions. Blead the brakes (many times). Checked vacume source and amount of vacume. Swapped the positions of the brake lines off the master. Adjusted rear brake shoes (many times). I'm pretty much at a loss here guys, short of throwing the whole mess out and starting over. Summer is coming and I'm not going to drive it this way another summer.

I've been told my brake booster is bad, when you step on the pedal and start the motor, the pedal does not move. That should not have an effect on the poor rear brakes. I'm wondering if the 9" drums and the corvete master are the problem or maybe the porporting valve is defective (its burried in a hole). I would like to get this right on one try, money is a little tight, I'll buy what ever I need....once more. I can live with out the power long as the brakes all work.
What ya think?
Gene

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#34347 - 02/09/07 05:57 PM Re: brake ratio? [Re: poorboy]
RodStRace Offline
I Live Here

Registered: 09/21/05
Posts: 12389
Loc: Chino Valley
Poorboy, sounds like you have a couple issues; hard pedal that does not 'sink' when vacuum assist starts, and little or no rear brakes.
1. Make sure there is vacuum to the booster, and that there are no leaks. Pull the hose off and hook up a vacuum gauge. Start the motor and let us know how much vacuum there is. Next note the idle with the hose still plugged, then connect it to the booster. Same idle after a minute? If not, it's leaking. Also lay under the dash and operate the pedal. There should be a slight vacuum sound when the pedal is pressed.
2. Before you go out and buy expensive pressure gauges, try to cobble up a bypass for the proportioning valve. A couple of double female fittings and a line with fittings should do it. Try that carefully to see if the rears work better.

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#34348 - 02/09/07 06:49 PM Re: brake ratio? [Re: RodStRace]
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
poorboy, what size is the master cylinder bore ? the wagner book i have shows most corvettes have a 7/8 or 11/16 bore. this should be ok for your application. is it possible your rear flexline[rubber hose] mite be partially collapsed ? i would be concerned about that firewall flex. also, i agree with rodstrace you should remove the proportioning valve & see what happens. is there any possibility of binding in any of the pedal assembly ? brakes can be a real pain sometimes...........

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#34349 - 02/11/07 04:21 PM Re: brake ratio? [Re: moparx]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
Sorry it took so long to respond, life got in the way...

There is vacume to the booster, as far as checking the rest, the car is sitting outside in a snow bank right now, the joys of running a business out of a 2 car garage. It will also be pretty hard to listen for a vacume noise under the dash, the car's exhaust dumps under the running boards and even with mufflers, it will be too loud to hear a vacume noise, unless its huge. I can tell you that when you lift your foot from the brake pedal, it takes a few seconds (like across an intersection) for the pedal to return to the top stop (and shut off the brake light switch). If you hook your foot under the pedal and lift, it feels like there is some resistance to it returning. I'm pretty sure the brake booster is junk. According to the web site I bought the brake assembly from the master is suppose to have a 1" bore. Frankly, I didn't measure it, but I will also be doing that. Another thought, the disc brake calipers are from a Chevy pickup, I'm wondering if they have a larger diamiter piston then the Corvette would have had. I also have no idea what size the rear wheel cylinde diamiter is, maybe everything is just sized wrong? (I've been reading a little) Life was sure a lot easier when I used to get all the stuff from the same car, stupid "kits" anyway.

I remeasured (as best I could) and the pedal from the pivoit to the center of the brake pad is 15" long and the arm that applies the pressure to the master is 3 1/2" long from the pivoit to the bolt hole center. If I figure it right, that would give me a pedal ratio of about 4,2 to 1? Wouldn't that require some extra foot presure to apply the brakes? I will be pulling the car into the garage in a week or so to get things ready for summer, so I will be pulling the pedal out to reinforce the firewall and will get a good measurement then. Until a few weeks ago I didn't know the firewall was flexing. My car still has the bolt in floor pans so there is little support at the bottom of the firewall. I plan on welding the top floor pan in instaed of having it bolt in and I think I can add a 1/8" x 1" strap standing at 90 degrees to the floor/firewall joint and still clear everthing. That should give me the stiffiness I will need.

I know its not a sign that things are right anymore, but all the brake hoses were put on the car new last spring when I put this mess together.

As I understand it, the porportationing valve also has the brake residual valve in it, if the guy sent me the wrong valve and I got one from disc/disc instead of disc/drum, could that be my whole problem with the rear brakes?

As you see, my mind is trying to cover too many variables. I have over $1000 in these brakes and I sure can not afford to do all that again!

At this point, I'm thinking I will reinforce the firewall, modify the pedal ratio to somewhere around 7 to 1, and trash the booster (its ugly anyway). I'm going to measure the bore diamiter of the master, the wheel cylinders and the brake calipers, and see how those look. I may also go to a junk yard and get a porportationing valve from a 5Th Ave and trash this one I have now. I do plan on doing one thing at a time, so I know and can let you know what the cause of the problem is.

I would still like your imput on where to begin. The firewall is a diffinate, the rest is speculation right now.
Thanks guys. Gene

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#34350 - 02/12/07 06:23 PM Re: brake ratio? [Re: poorboy]
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
from your discription of the pedal action[slow to return & showing some resistance] i think the booster has problems. as for the pedal ratio[4.28 : 1] it is in the ball park for power brakes. 7:1 would be a good ratio for manual brakes. not sure on the chevy truck calipers, but if they are close to the vette [4 piston ?] area size wise, this shouldn't be a problem. if the proportional valve also has residual valves in it, that could be a problem for two reasons. one, with your master mounted on the firewall, you don't need 'em[residual valves] as they are for use when the calipers & wheel cylinders are above the master mounting height to prevent fluid drainback from the cylinders & calipers into the master. 2nd, if in fact they[residual valves] are for a disc-disc setup, they would be[should be] a 2lb. valve[s]. are the brakes retaining heat, as in not releasing fully ? this could have glazed the pads & linings. just thowing out things here that may be of help. brakes can SURE be a pain at times......

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#34351 - 02/23/07 09:24 PM Re: brake ratio? [Re: moparx]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
Man, I finally got caught up with work a little and we got a short break in the weather earlier this week. I played a little with the coupe.

First thing I did was pulled off the master and redid a bench bleed, just to be sure both ports were delivering fluid. Seemed like there was a little air trapped in the port closest to the brake pedal, but after a few pumps both ports delivered good fluid. Uppon reinstalling the master I noticed that the vacume to the booster was cocked a little and was binding against the master. I had the vacume port entering at the top of the booster like Mopars do. I was able to flip the booster 180 degrees, so the vacume port is now under the master and it now clears the master compleately. All I was able to do after that was tighten the lines (working alone sucks sometimes) and hope for another day to rebleed the brakes with some help. Out of courisity I fired up the motor to check if the pedal pulled away when in motor started, it does not. I also varified that there is indeed vacume to the booster, and if the pedal is depressed there is a noticable change in engine rpm. That change remains until the pedal is released.

The thing I call a porportationing valve they call a "combination valve" It is for a firewall mounted master with disc/drum brakes, but the place I got it from also offers a "combination valve" for a disc/disc brakes. I don't remember any numbers on the one I have....and I sure can't see a number on it in the hole its in.

Not sure if I have accomplished anything or not, but at least I was able to do something. Unfortunately, its snowing outside again and they are calling for 7"-14" of snow over the weekend. That will put the screws to working on the coupe until at least mid week. Gene

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#34352 - 02/25/07 03:54 AM Re: brake ratio? [Re: poorboy]
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
can you hear any vacuume noise inside the car when the pedal is depressed ? the rpm shouldn't change when the pedal is depressed unless there is a problem with the diaphram[induced vacuume leak]. not any progress on my humpback lately due to the cold-i just can't take unheated garages any more..............we're supposed to get another blast of snow & sleet today.....stoopid groundhawg........

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#34353 - 02/25/07 07:36 PM Re: brake ratio? [Re: moparx]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
With the mufflers right under the running boards, there is no way I could hear a vacume leak unless it was whistling, which I don't hear. The motor was cold and the choke was on because the motor was just started (it was about 30*) and there was a deffenate rpm change when the pedal was depressed. The motor was on fast idle and it increased in rpm probably a couple hundred rpm change that remained increased until I lifted my foot from the brake pedal. (sorry didn't even think of looking at the tach. ) Once I lifted my foot off the pedal, the rpm dropped the couple hunderd rpm it had increased, back to the original fast idle. I know the carb on the motor has some issues and a rebuilt replacement is sitting on the shelf waiting for mother nature to cooperate. I'll have to try it again when I can run the motor until it reaches opperating temp and then get down where I can put my head near the brake pedal, maybe I can hear any vacume leak.

Probably won't happen this week, we got 6" of very wet snow and then it rained and is snowing again now. The car is once again burried in a mound of snow. This winter sucks. Gene

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#34354 - 02/26/07 02:28 PM Re: brake ratio? [Re: poorboy]
Fig Offline
member

Registered: 04/13/05
Posts: 167
Loc: Torrance, Ca
I went thru some of this same stuff on my 37. I ended up taking off the 7" booster and making manual brakes. I changed the ratio from 5 to 6. In retrospect I don't think that the 7" boosters do the job (mine was not a dual). I also ended up taking off my adjustable proportioning valve.
_________________________
1937 Plymouth R/S Coupe 440/727

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#34355 - 02/26/07 06:20 PM Re: brake ratio? [Re: Fig]
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
i agree that maybe the 7" boosters aren't what they are cracked up to be. another thing that i think some forget is that the larger the rear tire[diameter & width],the more rear brake is needed to stop. sometimes an adjustable prop valve is a waste of time.fig, do you know what size[diameter]master you used ? what size front & rear brakes ?

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#34356 - 02/26/07 06:30 PM Re: brake ratio? [Re: poorboy]
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
Quote:



Probably won't happen this week, we got 6" of very wet snow and then it rained and is snowing again now. The car is once again burried in a mound of snow. This winter sucks. Gene



we got only a couple inches of sloppy wet stuff but i still hate it......no wonder ron left new york for california...........

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#34357 - 02/26/07 07:36 PM Re: brake ratio? [Re: moparx]
Mopar Ron Offline
master

Registered: 01/20/03
Posts: 7989
Loc: Escondido, CA. Ron Podsiadly,...
Quote:

..no wonder ron left new york for california...........




we had some really bad weather here this week, we got .18 of an inch of rain

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#34358 - 02/26/07 09:15 PM Re: brake ratio? [Re: Fig]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
Fig, Did you have the Corvette master also? How did you do the push rod into the master when you dumped the booster, was the master cylinder plunger deep enough to hold the rod or did you change out to master to something different?

Ron, Really sorry to hear about all that bad weather you had to endure. Been to California twice, rained nearly all the week I was there both times. Sunny California? yea, right. We only got about 6" of the really wet heavy snow over the weekend, but now they are calling for another rain/snow mix this Wed.
Gene

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#34359 - 02/27/07 11:17 AM Re: brake ratio? [Re: poorboy]
Fig Offline
member

Registered: 04/13/05
Posts: 167
Loc: Torrance, Ca
Yes, I used the Corvette m/c, I think that it was 1 1/8". I don't remember exactly how I did it but I made up a plate for the rod to go thru so it wouldn't fall out. I can take a picture of it this weekend if you want me to.

I live about two hours north of Ron, we had horrible weather last night. We got close to 1/4" of rain
_________________________
1937 Plymouth R/S Coupe 440/727

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#34360 - 02/27/07 06:54 PM Re: brake ratio? [Re: Fig]
moparx Offline
Dreaming of implants

Registered: 09/03/03
Posts: 8975
Loc: north of coder
Quote:



I live about two hours north of Ron, we had horrible weather last night. We got close to 1/4" of rain



i don't know about you guys............maybe we should start a "search & rescue"..........

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#34361 - 02/27/07 07:55 PM Re: brake ratio? [Re: Fig]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
Fig, Would much like a picture if you would be so kind. Gene

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#34362 - 03/07/07 02:05 PM Re: brake ratio? [Re: poorboy]
Fig Offline
member

Registered: 04/13/05
Posts: 167
Loc: Torrance, Ca
Sorry it's taking me so long to take the picture, I have to jack up the car because the m/c is under the car. I keep forgetting to do it . Sorry
_________________________
1937 Plymouth R/S Coupe 440/727

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#34363 - 03/07/07 10:52 PM Re: brake ratio? [Re: Fig]
poorboy Offline
master

Registered: 01/19/03
Posts: 6315
Loc: Freeport IL USA
That's OK Fig, The car is still sitting in the snow bank! Its suppose to get into the 40s this weekend, maybe I can dig it out then.....Gene

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