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Old 04-07-2012, 11:40 AM
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Disc brake Cadillac rear calipers

I recently put disc brakes on the rear of my truck. I'm now running 4 wheel disc. The fronts work fine. The rears appear to be not working well. I'm using 78 Cadillac Seville calipers with the ebrake. I'm running a 69 GM corvette MC for disc/ disc applications. The MC is mounted on the firewall above the calipers and I have a separate line from the MC to the rear calipers with a Wilwood Proportioning valve in the line between the rear brakes and MC. The ebrakes works fine when applied, but I've noticed that the rear rotor isn't showing wear with normal driving or barely stopping the wheel when the brakes are applied? I've bled the system several times and the pads are adjusted correctly to the rotors. I also have the proportioning valve adjusted to wide open with no restriction. I'm wondering if I should remove the proportioning valve all together and see if that helps with the rear calipers working better. Any suggestions on what to look at?

Last edited by kleen56; 04-07-2012 at 11:47 AM.
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Old 04-07-2012, 12:15 PM
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Does the truck stop in a straight line? Does it stop as it should as far as distance? If yes to both, probably OK - but ...... those calipers may not be matched as far as fluid capacity with the fronts (whatever they are). If you have a 1" piston in the fronts, it will take less fluid to move a distance then say a 2" or multiple pistons on the rears. A proportioning valve will only modify the pressure about 55-60% so wide open - no change. As far as percentage of braking - a pickup may be as bad as 70/30 though 55/45 or 60/40 would be normal. Using wear as your indicator - the backs on my '31 with identical Wilwood calipers on all four corners show no wear with just a minimum of markings on the rotors
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Old 04-07-2012, 02:09 PM
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The truck stops nicely and in a straight line. The front calibers are GM stock calibers (not sure the year?) that came with the front conversion kit from Classic Performance Parts. I'm assuming that the rears should at least show some wear on the rotors if the pads are working. I understand the front has more stopping power which is evident in my case. I guess my question is how do I get the rear calipers to apply more stopping that what they actually do now. I'm assuming that the Proportion valve being wide open would allow the most pressure of brake fluid to the calipers? I appreciate the info, it's good info...
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Old 04-07-2012, 04:09 PM
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According to the Rock Auto Parts catalog, the Seville normally has 2-15/16" pistons for the fronts while the tried and true '80s GM Monte Carlo size car has a 2-1/2" piston. The Sevilles's are then 7/16" larger in diameter. I know you said that you have the e-brake calipers, which are supposed to be 2-1/2 - maybe - as it does look like there are some with the 2-15/16 rears just to confuse the issue

If you have the 2-15/15 version, this calculates as each front piston having 4.9 sq in of surface area with the rears at 6.77 or about 25% (round numbers) more surface area. A physics law(Boyle's) - "Pressure and Volume are indirectly propotional to each others. if you increase the Area the pressure will be decresed, and if you decrease the area of the applied pressure, the pressure will be automatically increased. Hence, pressure is indirectly propotional to area."

This tells me you might have a problem in that the rear caliper pistons may be too large and not giving you enough brake power. In actuality, those calipers should probably be in the 2 to 2-1/4 diameter range

Can you tell - I'm being cautious. Too dam' many variables when you deal with OEM 'stuff' adapted to our hobby cars (I really like my aftermarket Wilwoods )

Dave W
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Old 04-07-2012, 04:34 PM
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I read some where

that rear disk brakes only add 22% to the total braking with a front engine vehichle. If thats the case I dont see where it is an advantage over brake shoes that don't drag all the time thus giving better gas mileage!! maybe in all out racing rear disks would be an advantage. There is a cost savings here also... If all the above is correct
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Old 04-07-2012, 05:37 PM
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Quote:
Originally Posted by rsfyj
that rear disk brakes only add 22% to the total braking with a front engine vehichle. If thats the case I dont see where it is an advantage over brake shoes that don't drag all the time thus giving better gas mileage!! maybe in all out racing rear disks would be an advantage. There is a cost savings here also... If all the above is correct
The reason I went with disc brakes on the rear is simplicity. It's easy to work on disc brakes versus the drum brakes. I have had issues with seals leaking, brakes not working in water, etc. The disc system is simply more superior to the drum system and it wasn't that expensive to do the conversion.
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Old 04-07-2012, 05:42 PM
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Quote:
Originally Posted by Irelands child
According to the Rock Auto Parts catalog, the Seville normally has 2-15/16" pistons for the fronts while the tried and true '80s GM Monte Carlo size car has a 2-1/2" piston. The Sevilles's are then 7/16" larger in diameter. I know you said that you have the e-brake calipers, which are supposed to be 2-1/2 - maybe - as it does look like there are some with the 2-15/16 rears just to confuse the issue

If you have the 2-15/15 version, this calculates as each front piston having 4.9 sq in of surface area with the rears at 6.77 or about 25% (round numbers) more surface area. A physics law(Boyle's) - "Pressure and Volume are indirectly propotional to each others. if you increase the Area the pressure will be decresed, and if you decrease the area of the applied pressure, the pressure will be automatically increased. Hence, pressure is indirectly propotional to area."

This tells me you might have a problem in that the rear caliper pistons may be too large and not giving you enough brake power. In actuality, those calipers should probably be in the 2 to 2-1/4 diameter range

Can you tell - I'm being cautious. Too dam' many variables when you deal with OEM 'stuff' adapted to our hobby cars (I really like my aftermarket Wilwoods )

Dave W
Thanks Dave.
The reason I'm going with the Cadillac calibers is the ebrake system which is easy to use and adapt to my stock truck ebrake system. I was thinking about going with the stock calibers like the front and be done with it and not utilize an ebrake at all. I'm sure that would fix my issue. The other option is going with wilwood or aftermarket caliber which are expensive but work nicely. I may also check into a line lock type system that locks the rear brakes when applied which I could use as a ebrake and would allow me to use standard front calipers. The Cadillac calibers weren't cheap either and it would be nice to have them working properly. What's the purpose of having 4 wheel disc if they don't work... Thanks for your help and advise!
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Old 04-07-2012, 07:26 PM
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If you do go with smaller calipers, the option is to use a pinion mounted parking brake. While they really aren't very good, they at least satisfy the letter of the law and will at least hold somewhat. I've had this one on 2 cars, but I see they have a better one now:
http://www.tsmmfg.com/pinionmountedpb.html


Now - the BS of 22% - sorry, but there are better reasons to switch to discs. They seldom if ever will fade to uselessness, they will work just as well when wet, wear at a much slower rate, help fuel mileage by not contacting the rotor when not applied plus weigh significantly less, are easier to service, and additionally, shorten stopping distance by a LOT
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Old 04-07-2012, 08:59 PM
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just as a test-try putting the prop valve in the front half of the system
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Old 04-08-2012, 11:12 AM
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Well in my life

i have changed as many rotors, pads, and callipers as I have brake shoes. So to me having them in the rear is only a 22% high for my car.. And another added costs versus a set of cheapo brake shoes and I still get 22% how can you lose???
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