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Old 04-24-2010, 08:33 PM
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No Brakes - "And I don't know why!" HELP!

OK guys. I have a problem that has me completely stumped. My problem is that I cannot get any pedal resistance on my new brake system. All components are new or lightly used. I've attached two photos of the brake assembly for your benefit.
Here is some background:
I have GM disk brakes in front and GM drum brakes in the rear. I have an 8" GM power booster off a mid-size car like a Lumina. I have a reman GM dual master cylinder (1" bore) intended for a 1/2 ton GM pickup. Because the master cylinder is under the floor and lower than all the brake cylinders, I have included a 2 lb. residual valve in the front brake line inches away from the master cylinder and a 10 lb. residual valve in the rear line with a proportional valve between the master cylinder and the 10 lb. residual valve. I have a new vacuum line from the booster to the rear vacuum connection on the 4 brl. carb. and it is pulling good vacuum. I have bled the lines starting from the farthest wheel and worked to the closest wheel, doing the front first and then the rear. I have bled them each twice searching for any air. No air was revealed on the second bleed. The master was prepared on the bench exactly according to mfg. instructions and had only about 1/8" travel when operated by hand before it was installed on the car. There are no leaks anywhere. The rear drum brakes have been adjusted and rechecked twice for minimal clearance from the drums. I have 6 1/2" of pedal travel from full rest to the floor. The pedal ratio is 6 to 1. I'm using fresh DOT 3 brake fluid.
I cannot get a firm pedal, with or without the booster assist and I cannot figure out why. What am I missing or doing wrong?

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Old 04-25-2010, 01:35 AM
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Sometimes it all gets too hard...

Can you do a pressure bleed? This is where you put pressure on to the reservoir and pump fluid through the whole system?

Or can you pump fluid back through each bleed nipple back to the master?

It would seem you have a bit of air somewhere, unless you have really spongy flexible lines or something like that.
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Old 04-25-2010, 09:39 AM
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Ray,
Thanks for the suggestion. I do not have any means of pushing fluid thru the system.

One thing occurred to me however, I am using a 1" bore MC as I had used on the car prior to rebuilding the brakes. (Before the brakes were manual) I'm thinking maybe I should move up to a 1 1/8" bore MC to increase the flow. My concern is that the larger bore could be too much pressure and cause the brakes to become too strong and very touchy. I have a shop manual that provides me with the math to calculate the MC bore based on the brake caliper dimensions and brake line sizes, but it requires me to disassemble the brake calipers and measure the piston diameter and piston travel. I'm hoping I don't have to go to that extreme to resolve my problem.
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Old 04-25-2010, 12:38 PM
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You have air somewhere in the line ........ start off with right rear ..... take a length of aquarium ( clear ) hose ,,, that will fit over the brake bleeder nipple tight ... make a 3" loop with the loop facing up ,,, crack open the line and let it gravity feed .... bubbles will eventually come out ... you will see them thru clear line .. tighten line ,, pump-up,, crack line and you will see more .. 3-4 times and you will start to get a firm pedal .. works every time .. i alyways bleed brakes by myself .. the other mechanics that i worked with can't belive it .. and i've even offered them $50.00 if they found air .. still have my $50.00 ....
do this on all 4 .. r/r -l/r,, r/f -l/f ...
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Old 04-25-2010, 01:24 PM
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Bob,
Great looking 48!

I'll try your method. I assume the clear tube is dry when I start and I also assume the MC reservoir cover is loose to allow the fluid to flow naturally.

I'm also going to check the clearance between the MC primary piston and the booster plunger to be sure there is no excessive clearance.

One more thing. In the Knowledge Base section I was reading about the purpose of a Combination Valve (which I am not currently using) and how it could affect the fluid flow to the disk and drum brakes and result in a soft pedal. Based on what I read, the system needs one and so I am planning on getting one this week. I assume there is only one kind to buy. Right?
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Old 04-25-2010, 02:01 PM
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A larger master cylinder will reduce pressure, not increase it...
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Old 04-25-2010, 02:50 PM
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I just redone my brake system also. I had the same problem. Swore I was bleeding by hand enough. Well my neibor had a pump that draws the fluid through the system. I started at the right rear and let it suck fluid for 20 min. I did this at each bleeder, and to my surprise my brakes came back. After using the suction to draw the fluid I went back to each bleeder and did the pump the peadal method to make sure I had pressure . Now hope this helps you. I have other issues now that deal with vacuum/booster, which I'm working on it.
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Old 04-27-2010, 05:57 AM
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I didn't think the brakes could be gravity bled when the master cyl was under the floor. It could be a problem with the mc. I've read on here that some of the rebuilds are junk.

Moon
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Old 04-27-2010, 11:59 AM
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Quote:
Originally Posted by Moon
I didn't think the brakes could be gravity bled when the master cyl was under the floor. It could be a problem with the mc. I've read on here that some of the rebuilds are junk.

Moon
As others have said, you have air in the lines. So long as the M/C is higher than the calipers/wheel cylinders, you can gravity bleed. You may need to jack the car up and let the axles hang to get the bleeder ports lower than the M/C.
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Old 04-28-2010, 10:40 PM
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Might not be air in the lines.

Without being there to see it or know exactly how everything went together my best guess is the pushrod on the brake booster is not touching the plunger on the master cylinder. Had this happen on my Chevelle. I took the master cylinder off the booster and measured the forward travel and compared it to the travel needed by the master cylinder. In my case the plunger on the booster when at rest was sticking out by about a half inch. The plunger of the master cylinder had a hole that was about an inch and a half deep, the plunger even with the pedal all the way down wasn't even pushing on the master cylinder. No pedal at all. Hope this helps.
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Old 04-29-2010, 12:18 AM
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Under the floor mounted master cylinders typically have too high of a pedal ratio and not enough pushrod travel to use the entire stroke of the master cylinder. I've seen 6 or 7:1 pedal ratios on many aftermarket pedal/bracket assemblies, when it should be 5-6:1 for power brakes, and if you are using the stock pedal with an adapter for the master cylinder, it could be even higher.
Most standard GM passenger car master cylinders need a pushrod stroke of about 1"-1.2" to be able to bottom out the cylinder...They don't make full pressure in a properly designed system until you push the piston in about 5/8"-3/4" . A truck master cylinder many times needs even longer of a stroke...A mid 70's GM master cylinder for a 1/2 ton truck has about 1.5" of total piston travel...


If you are using a set of metric GM brakes, a 1" master probably does not have have enough volume to deliver enough fluid to get to where the brake system actually builds pressure. If you are using the early GM calipers, then I'm almost sure that MC won't even come close.

I'd check a few things.

Before you bleed the system again, check the clearance between the pushrod and piston, (that was mentioned before in this thread). Master cylinders intended for use with a manual pedal have that deep hole in the piston, ones for power brakes have a shallow hole..Mant aftermarket master cylinders come with a spacer plug, that goes in the piston for power brakes,, so you can use them with either manual or power setups.

Check the available stroke at the pushrod, you should have at least 1" of pushrod travel when you push the pedal through it's entire stroke. Numerically high pedal ratios, thick floor mats, and modified pedals can all decrease the travel.



I'd go with the 1 1/8" passenger car master cylinder myself, the pedal might be a little stiff, but that seems to be the one I have installed the most in the early, under floor master cylinder setups with disc brakes...

If you are using metric GM calipers, check to see that you don't have the low drag type. The low drag type calipers use up a bunch of pedal travel, just to get the pads near the rotors, and that in itself can make for a no brake situation..There is an easy way to check, see this thread: Test for low drag calipers



You may have any or all of these issues, my bet is that you have too little pushrod travel, and too small of a master cylinder.

A combination valve will not help get you any more pressure from a system that won't make pressure in the first place.


Later, mikey
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Last edited by powerrodsmike; 04-29-2010 at 12:27 AM.
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Old 05-03-2010, 09:09 AM
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Powerrodsmike,

I measured the clearance between the booster push rod and the MC. The clearance dimension is 0.0625" +/-. The booster pushrod travel is 1.25" maximum. I vacuum bled the system last Saturday and saw no change in the results. I have ordered a NEW MC designed for power front disk and power rear drum brakes for a 1987 1/2 ton Chevy pickup. It has a 1" bore. My front disk brakes are GM single piston calipers and the bleeder requires a 10 mm wrench to open it. I do not know how to determine if the calipers are metric. I bought them as a package from Fatman Fabrications. In the past the brakes worked fine using a manual MC. My winter project has been to replace the manual system with a power assist system. All else has remained the same. I cannot gravety bleed because the MC is lower than all the wheel calipers. When we vacuum bled the system we started at the farthest corner and worked our way back. We bled until no air was observed and then went back and did it again to be sure. I'll post the results when I install the NEW MC. Our suspicion is that the second reman MC is also no good. That is why we are going to a new MC.
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Old 05-03-2010, 11:04 AM
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If you replaced all the calipers. Make sure you put them on the correct side. The bleeder must be at the top. If you put them on the wrong side the bleeder will be on the bottom, the fluid will run clear but there will still be lots of air above the fluid.
This is a common mistake I have seen many times.
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Old 05-05-2010, 11:09 AM
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T-Bucket 23,

All caliper bleed screws are at the top. I ordered a new MC and it should be in today. Hope to have it bench bled and installed by Saturday. This should hopefully eliminate all my problems.
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Old 05-05-2010, 08:37 PM
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I have discovered my problem. When I designed my new brake mechanism to deliver force to the booster via my old brake pedal, I designed the pedal ratio in the wrong direction. I designed a pedal ratio for 9:1 rather than 4.5:1. DUH!!!! So tomorrow I will remove the mechanusm and correct the design. It should only take an hour or so to make the fix. DUH!!! Well, live and learn. That is part of this hobby too! I'll post the results after I make the changes. Boy, do I feel stupid!
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