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HELP..... Disc upgrade problems

Leper

Leper

1/2 ton status
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Feb 18, 2005
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Dallas Baby!!!!
I still have a very low pedal and not much braking power. What has been done so far on a 79 Blazer. DIY4X rear caliper mounts. Reman calipers, proportioning valve from Shane at blackwidowperformance,3/4 ton MC, lots of bleeding. One of the front calipers is a newer reman, the other is older. Even when you pump the pedal, the braking force does not increase. The pedal never gets hard. The pedal is a little higher without the engine on, which is normal. The booster makes a little noise when you push the pedal, but I am under the impression that if the booster went out, the pedal would be harder to push. The truck does not dive to one side or anything. I need help guys. I am out of ideas.
 
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Sorry if you've already posted this info, but is the MC new since you started bleeding it?

I've had a couple of "good" MC's that were fine, right up until the point I needed to bleed them, at which time I could never get a firm pedal.

I guess the question needs to be asked, do you have spongy brakes, or is it just a low pedal and not much work being done by them?

You say not much braking power, I assume this means it takes longer than you think to stop, as well as not being able to lock them up at all?

With the truck up in the air, can you stop the rear wheels with the brake pedal?
 
New as in just got it. I know the problem with bleeding brakes on an older MC, you can push the piston farther than normal and the corrosion/rust on the rarely used part of the piston can damage the seal. And yes, I did bench bleed the MC prior to install.

I went back to the stock MC and the problem still exists.

With the wheels off of the ground the brakes do stop the wheels. The pedal is lower than normal, but it doen not feel spongy. If you put the pedal to the floor, the truck does eventually stop, but it takes a bit. None of this was present before the swap to rear disc.
 
stock reman 3/4 ton gm calipers and new 3/4 ton rotors. 14bff with diy brackets.
 
Leper said:
I went back to the stock MC and the problem still exists.
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So you can pretty much rule out any master cylinder issue I'd say. Any noticeable difference whatsoever between the stock and other?

Is your booster actually assisting, or could the low pedal be making it difficult to figure out that it's not?

Easy test for the booster is to start the truck, let it idle for a couple of seconds (don't touch the brakes) turn it off, let it sit for awhile (45 minutes is the GM spec I saw) then get back in, and see if you've still got power assisted brakes for a couple of applies with no engine.
 
sweetk30 said:
ya but what axle in front? the 60 calipers are masive to a 3/4 ton set.
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front is an 8 lugged 10 bolt.

i dont think we drove with the k20 MC installed, but bleeding and pumping in the driveway, there wasn't a discernable difference in pedal feel.

the booster is assisting plenty.to give you an idea whats being felt... with the truck running(idle in driveway) the pedal will go to the floor with just slightly more pressure than you would use to come to a normal stop from 40 mph per hour or so.

after shutting off, there is 1 fully assisted pump then not much after that.
 
Try adjusting the pushrod as short as it will go. Try that, and progressively get longer, testing pedal feel after each adjustment??? I have had problems with Fords after changing MCs around, because of different lengths of pushrods being required for different MCs.
 
yes, we've bench bled a total of 3 times now (twice with stocker, once with k20)

i think ill have to look into the pushrod as well. thanks guys.
 
3 on the tree said:
Try adjusting the pushrod as short as it will go. Try that, and progressively get longer, testing pedal feel after each adjustment??? I have had problems with Fords after changing MCs around, because of different lengths of pushrods being required for different MCs.
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Pushrod is not the issue. The freeplay in the pedal before pressure is the same between the 2.
 
Just for grins; bypass the prop valve and hook up front brakes and try them. Then just hook up back brakes by themselves and try them. Maybe you can isolate the problem-front or rear.///
 
3 on the tree said:
Just for grins; bypass the prop valve and hook up front brakes and try them. Then just hook up back brakes by themselves and try them. Maybe you can isolate the problem-front or rear.///
Click to expand...

How would you do that?
 
I would like to know any info on this as well... I just did my rear discs with a 60 upfront, did the same as Leper and got a 3/4 mc, and I have the same problem he is describing. Low pedal, difficult to stop until the pedal reaches the floor, and the pedal is not firm. I have not tried anything really to correct it yet, as far as proportioning valve...adjustable brake lines...etc. So it would be nice to see what some of yous suggest.
 
Leper said:
How would you do that?
Click to expand...
Make up or buy some short brake lines that connect the MC to the brake lines without going through the prop valve. Get brass plugs to screw into whichever MC port you are not using,ie front or rear. I personally think that you are not getting enough flluid displacement for the rear brakes, and that is making the whole system spongy. Isolating each set of brakes and testing them will probably reveal that the fronts are fine, but the rears are not. Aren't brake upgrades fun????
 
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