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Old 01-31-2006, 07:07 AM
Too Damn Slow
 
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Serious brake problems

My braking system is all messed up. I need help finding what parts I need to buy.

This is in a '90 S-10 Blazer with a 454 big block

I have a stock factory master cylinder with a 1" bore and the correct pedal ratio. I am not using a combination valve. The front line(rear of MC) goes to a tee and out to SSBC Force 10 calipers sqeezing factory rotors. The rear line(front of MC) goes through a Wilwood adjustable prop valve to Ford 11X2 drums.

The problems are numerous. The rear brakes do not work at all. I've been driving it for a while and the shoes look brand new. The front brakes do not work very well either, but the pedal is extremely firm. The front brakes sometimes seize up, especially the front right. Sometimes the pedal will get so hard I can't even push it and I can hear the fluid boiling in the calipers.

I'd like to switch back to a power setup if a manual can't cut the mustard, but am unsure of the idle vacuum I will have with my new engine combo. What do you guys think? 454 BBC 9.5:1CR w/ Victor Jr intake and Voodoo 282/290 adv dur, 231/239 @ .050 dur, .600/.600 lift on 110LSA. I just want to fix this once and for all. I think a medium sized booster will something around 8".

HELP!!!!

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Old 01-31-2006, 09:22 AM
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It sounds to me like you are holding high residual pressure in the front lines. Your pads aren't backing off when you let up on the pedal and the resulting heat from friction is boiling the fluid. The expanding fluid is taking up all of the travel in your MC causing the hard pedal.

This may be due to the wrong fluid. Are you using DOT5 disc brake fluid? Too low of a DOT number will allow the fluid to boil at a very low temp.

Is there any pedal travel when it's cold? If not there is a mechanical problem in the system. You may have a bad residual valve or too high of a psi rating on the one you have.

Your rear brake problem may just be a product of the MC piston not moving far enough to apply pressure due to the front system being overpressured. I'd fix the front brakes first then back off the rear proportioning valve to allow full pressure. Then do a few stops on loose stone to see if you can lock up the rears. Adjust your rear pressure from there.

Raise the front wheels off of the ground when it's cold and see if the wheels spin freely. If you have to force them you have too much residual line pressure. If they spin freely cold then won't after you have driven it I'd switch to a high temp brake fluid.

My answer is just basic stuff and I'm sure there are others here that are much wiser about custom brake systems but this will give you a place to start.

Tom
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Old 01-31-2006, 03:09 PM
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Hi. Like 39Deluxe said, get both front wheels off the ground and see how easy they turn. If they are tight I would suspect the mc isn't releasing fully after each pedal push. if they are tight, disconnect the rod that goes from the pedal to the mc and see if the front wheels turn easier. If they don't and since you aren't using a combo valve I would then suspect some type of caliper/rotor misalignment problem. If they turn easier you may have to shorten the rod. Hope this helps.

Moon
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Old 01-31-2006, 04:46 PM
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DOT 4 fluid should be adaquate for the temps unless racing conditions, DOT 5 is silicone rather than polyglycols, and way more expensive and also, even in this case dosn't matter, cannot be used in ABS systems. Also remember that DOT 3 and 4 brake fluid is hydroscopic, and water in the fluid could cause the fluid to boil due to the lowered boiling temps.
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Old 01-31-2006, 08:47 PM
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Is the MC a step bore? Does the line to the rear have a residual pressure valve in it. it should have a 10#. Are you sure that the fronts do not have the residual pressure valve in the line. Those RPV's can be inline or in the MC. Are the lines plumbed to the correct ends of the MC. Did the truck have a booster on it originally, is it still there? You said the pedal ratio is correct, what is the ratio? Is the pedal allowing the MC to retract all the way? When you bled the brakes did they bleed freely? if you have no combination valve then you should have a metering valve (also called a "stayoff valve) in the front brake line to keep the pressure off of the calipers until the pressure reaches 100 -150psi. this is so that the fronts are not doing all of the work at low speeds. those metering valves are in almost all disc/drum systems that I know about. ECI sells them. They do help to keep the rotors cool when you are driving stop and go. They let the back brakes begin to work first. Drum brakes require at least 75 psi to start braking action. It sounds like the fronts are dragging. The fluid should not be boiling. Not even if you have the cheapest fluid known to mankind, unless you are racing on a tight roadcourse. If it boiled just due to normal driving then it would not meet requirements that the DOT specifies. If you have 1 side doing things that the other side isn't doing then I would look at mounting misalignment or a frozen piston or caliper slider. Typically misalignment will give a spongy pedal but your brakes are so mutated that I wouldn't rule anything out. what is the total piston area in those calipers and how different in area are they from the stock calipers. As far as a booster, the 8" might work ok, you may need a vacuum pump. You could go for a (ugh) hydroboost if you have a power steering pump to plumb it to. I think I would try to get your big issues sorted out before you start thinking about a booster. hope this helps, mikey
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Old 02-01-2006, 06:50 AM
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I am using good fluid. I think my biggest problem is the lack of a combination valve. I will pick one up and check for preload on the master cylinder pushrod. I am going to check for clearance around the headers also to make sure I am not boiling the brake fluid there too. Now I just need the snow to melt so I can work on it.
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