Brake Caliper reducer for RDB???

1Fine40

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Has anyone tried using a brake caliper piston reducer for the GM rear disk brake conversions? (http://www.colemanracing.com/catalog/index.php?cPath=28_1392) Seems to me that if you were to reduce the size of the piston, you would reduce the force exerted, and make it less biased. I have talked to a few rdb converts, and they say thay even with a Wilwood proportioning valve, their rear brakes are still too strong. Anybody try this?

TIA!
 

dgangle

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1Fine40 said:
Has anyone tried using a brake caliper piston reducer for the GM rear disk brake conversions? (http://www.colemanracing.com/catalog/index.php?cPath=28_1392) Seems to me that if you were to reduce the size of the piston, you would reduce the force exerted, and make it less biased. I have talked to a few rdb converts, and they say thay even with a Wilwood proportioning valve, their rear brakes are still too strong. Anybody try this?

TIA!
Short answer: I have bought it but not installed yet. I have the same symptoms you describe.

Long answer: I have a 08/75 FJ40 that came with 4 wheel drum brakes. I converted the fronts to 79 minitruck disks with the larger V6 4X4 truck calipers and LC vented rotors. My rears are the standard Monte Carlo calipers on Chevy truck rotors with Profitt's brackets. I have unresolves proportioning issues. I have tried a 09/75+ LC disk master and booster, adjustable proportioning valve and most recently an FJ80 non-ABS 1" bore mastercylinder from Marlin Crawler. Nothing will get the fronts to lock before the rears, period. My last resort is the Coleman Monte Carlo caliper piston reducers. I have run all new brake lines and bled 500 gallons of DOT4 through it. In a panic stop situation the rears lock causing the truck to slide sideways. Add a little rain and coming down a grade and it will swap ends. I have discussed this with a number of people who have the same set up as me and they don't have the same issues, even with no adjustable proportioning valve. I am really getting tired of throwing $$ at this problem. I promise to post my results.
 
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1Fine40

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Maybe I could rephrase this post to ask if anybody is NOT having weird issues with rear disks.....and what did they do different than the standard install? I assume most people are using a prop. valve and crank it down a bunch right???
 
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1Fine40 said:
I assume most people are using a prop. valve and crank it down a bunch right???



Ahhhhh, well then, that might be the problem. This is not a lefty loosey-righty tighty deal at all. Screwed all the way OUT ie, counter clockwise, gives you maximum restriction(57%) to the rear circuit. Screwed all the way IN gives you full flow to the rears.
That is a Wilwood p/n 260-8419


Ed
 

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dgangle said:
Nothing will get the fronts to lock before the rears, period.
Gangle-


Because of weight transfer, I highly doubt that you will get the front to lock before the rear, no matter what combo of master/booster/proportioning valve/caliper/etc. you decide to use. I have not ever been able to get the fronts to lock up on pavement, with a new master/calipers and 31” tires on a stock 40 series, going forward. They will lock up going in reverse, but again, that is due to weight transfer. I do not think that having the braking system set up so that it will lock the tires up and cause the tires to skid would be a good thing, front or rear. Once you start skidding, you loose your braking ability/efficiency, and the ability to control your direction, especially in the front.

This problem exists with a STOCK 12/82-40 series that has drum brakes in the rear. The rear drums lock up the 31” tires in a panic stop situation, just fyi.



Do you still have a manually adjustable proportioning valve in the system?


Where is it set? How far is it turned in or out?


You will get an increase in front braking if you upgrade to the ’91 V6 4Runner calipers. The Yellow truck runs those up front with his stock Toyota disc brake master cylinder and just like many other people out there, has noticed an increase in stopping power. He does not run a proportioning valve in his system, but also runs a 38.5” SX Swamper.


The metric, or small GM caliper, or Monte Carlo/Eldo caliper is not really up to stopping that much rotating mass anyhow. That has been my experience with brakes in the Red truck, and why I only ran the small metric caliper in the rear of my truck for a couple months, before fabricating new caliper brackets for the larger GM caliper.
 
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After a year of not knowing my boosters body was severly fatigued, I have the same problem with the rears. Everything, besides the rear hard line, is new. I even tried running a stock toyota pickup proportioning valve inline of the willwood to no avail. In my neck of the woods we have steep inclines with loose cover on hardpack. A hard application of the brakes can make you spin harder than a speed freak in Barstow (local culture, unfortunately). Haven't had the rear fling out on me yet though. Wilwoods should be labeled with arrows and decrease/increase. Plain old summit racing ones and empi's (all almost identical) work in the same fashion.

Now I'm not sure if the smaller piston is really going to solve all of that. Now a combo of a smaller piston with a decreased effective rotor diamater would do the trick. My piece of s*** jto caliper brackets do the opposite, the outer edge of the brake pad sticks out past the caliper. And this is on my 55, which has a much different weight distribution than a 40. But, drumroll please..... my system acted very similar with my drum brakes back there and v6 calipers
 
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Can you give more info on the larger GM calipers-donor vehicle,years,will it fit on the same rotor as the Monte caliper? Now collecting parts to do 4WDisc and thought that a proportioning valve took care of balancing front to rear discs until reading the above posts. When the brakes are fixed my 71 will be a DD for my kids transport to the local community college and I want it done RIGHT the first time, for their safety. I don't have calipers for the front yet, but I do have the GM rotors. Any info will be really appreciated. THANKS.
 

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Handcannon-


You will not want to go with a larger caliper, unless you plan on running a 42” tall tire. That is what I am using on my Red truck, along with the non-Toyota axles. I am sorry that I was not clear about this in my previous post.


Sorry.
 

dgangle

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Poser said:
Because of weight transfer, I highly doubt that you will get the front to lock before the rear, no matter what combo of master/booster/proportioning valve/caliper/etc. you decide to use. I have not ever been able to get the fronts to lock up on pavement, with a new master/calipers and 31” tires on a stock 40 series, going forward. They will lock up going in reverse, but again, that is due to weight transfer. I do not think that having the braking system set up so that it will lock the tires up and cause the tires to skid would be a good thing, front or rear. Once you start skidding, you loose your braking ability/efficiency, and the ability to control your direction, especially in the front.

This problem exists with a STOCK 12/82-40 series that has drum brakes in the rear. The rear drums lock up the 31” tires in a panic stop situation, just fyi.

Do you still have a manually adjustable proportioning valve in the system?

Where is it set? How far is it turned in or out?

You will get an increase in front braking if you upgrade to the ’91 V6 4Runner calipers. The Yellow truck runs those up front with his stock Toyota disc brake master cylinder and just like many other people out there, has noticed an increase in stopping power. He does not run a proportioning valve in his system, but also runs a 38.5” SX Swamper.

The metric, or small GM caliper, or Monte Carlo/Eldo caliper is not really up to stopping that much rotating mass anyhow. That has been my experience with brakes in the Red truck, and why I only ran the small metric caliper in the rear of my truck for a couple months, before fabricating new caliper brackets for the larger GM caliper.
Steve,
Thanks for the info. Let me clarify. My whole point about the front/rear lock up is that the rears lock up first which causes some real oh-s***s in a panic/wet/downhill situation. Obviously, it is not right. I would much prefer to have the breaking forces even or biased towards the front, don't you? I agree on the skid/loss of control issue but the rears locking up first is just plain dangerous.

I have upgraded to the larger V6 4X4 truck calipers and vented rotors. I originally bought the even-bigger V6 4Runner calipers but they wouldn't clear my MT wheels. So I went down a size to the V6 truck calipers. Still, they are considerably larger than the stock LC or mini calipers.

I do not understand your comment about the small GM caliper not being up to stopping that much mass as related to my problem. My problem is they are working too well w/ 33's!!

I appreciate your help. I didn't intend to steal the thread but only to 'me too' on the problem and state I plan to do the Coleman piston reducers as the next step in this money pit otherwise known as 4WDB.
Dave
 

1Fine40

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Ed sez:"anybody else with a proportioning valve screwed it the wrong way?"

Actually, when I said cranked down, I meant it was a reduced to the rear as it can be....the full 57%..I DO read instruction sheets :) From these responses, I can see I'm not totally alone in this problem. What about adapting a Toy mini prop valve to a Cruiser rear? They depend on weight and adjust as the suspension compresses or extends. Anybody thought of trying that? Appreciate the comments!

Oh duh, I should read twice before pulling the trigger on a post........the Toy mini thing seemed like a winner........:(
 
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dgangle said:
I do not understand your comment about the small GM caliper not being up to stopping that much mass as related to my problem. My problem is they are working too well w/ 33's!!


No worries Dave...


This was in reference to the 38.5"s...not a tire smaller than 35"s...


When I changed out my rear axle, I went to the smaller rear caliper, as that is what Warn provided caliper brackets for. I ran this for about two months, and then made new caliper brackets for the larger GM caliper, as the smaller caliper did not seem to provide the stopping power that the larger one did, with the 38.5"s that I was running, and then the 42"s.


Sorry about the confusion.


-Steve
 

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1Fine40 said:
Has anyone tried using a brake caliper piston reducer for the GM rear disk brake conversions? (http://www.colemanracing.com/catalog/index.php?cPath=28_1392)
I installed the caliper reducer
kits and foud a couple of things that didn't work out too well so I
have removed them. I never did actually run them on my cruiser. The
first issue I ran into were that the piston reducers spaced out the
piston side pad so that the gap between a new set of pads installed
in the calipers was about 21mm. The rotors I am using from a mid-90's
GM truck are 24mm thick. Therefore, the caliper would not fit over
the rotor unless I shaved material from the pads.

The second issue was that the stock piston dust shield "bellows"
(circular piece of fan-folded rubber) would not stay seated on the
new piston. There were no machining groves in that area like that of
the stock pistons which help to retain this dust shield. I was afraid
that over time the caliper's piston walls would colect water and dirt
and who knows what else and eventually cause a brake failure.

The idea is still good and valid in my mind if someone had the tools
and knowledge to make some modifications for our particular street
application. As of now, my kits are collecting dust on a self in my
garage.
 

dgangle

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Very interesting. I will pay attention to these items and see what happens as I have no apparent alternatives. So what did you end up doig to correct your front to rear brake bias problems?
 
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WAG here, but couldn't you grind off some of the friction surface on the pads?

Less surface=less friction=not as much braking.

Ed


WAG, like I said
 

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dgangle said:
Very interesting. I will pay attention to these items and see what happens as I have no apparent alternatives. So what did you end up doig to correct your front to rear brake bias problems?
Prop valve. Running it biased all the way to front brakes. Its not the perfect setup, wish it was better, but it works better than the drum brakes.
 

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Degnol said:
WAG here, but couldn't you grind off some of the friction surface on the pads?

Less surface=less friction=not as much braking.

Ed


WAG, like I said
I rthought about trying this but haven't. One problem with doing this might be that the smaller pad surface would still get the same total pressure applied from the piston so the smaller pad area now has more pressure per square inch countering at least some the effects of the reduced pad surface.
 

dgangle

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force=pressure X area

or typically

LBS=PSI/in2

To reduce the force you must either reduce the pressure (prop valve) or area (piston diameter). Like Mike says if you made the pads smaller they would just wear quicker as they are still getting the same force from the pistons in the caliper. I have the prop valve taking the highest pressure drop available (all the way out). This is where the smaller pistons come into play.


As a side note I previously installed two proportioning valves in series. The combination of the two took too much pressure drop making the truck not want to stop at all. I learned usually anything below 600 psi (drums) & 800 psi (discs) will not stop. so I am tring to find a happy medium. I think the smaller pistons with the adjustable prop valve will be the ticket.
 
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