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Old 12-23-2005, 09:31 AM
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Brakes that don't brake

Well, here I am 2 weeks later, frustrated, a couple of hundred bucks poorer and still no brakes! We (a couple of cruise buds and I) have put about 20 hours on this with no luck! So, I come to you for more help by starting over with a new thread.

The car is a 39 Ford 2D Sedan Street Rod. It was built on a custom made Fat Man rolling chasis that included a 4 wheel disc brake system. The master and booster are frame mounted below the plain of the caliper pistons on brakets and pivots that are part of the purchased chasis. The rotors are 9". The wheels are 20" rear and 17" front.

The booster was a 7" single chamber. The master was a late 60's, early 70's Chevrolet (Corvette type). The inside of the master cylinder had been painted(!) and, of course, the brake fluid had softened the paint and made a mess. So, we replace the master cylinder with a new one (same type) a installed a new 7" dual chamber booster. We used a vacuum gauge to check for adequate vacuum to the booster. We checked the horizontal travel of the push rod on the booster and the actuating rod on the master and it looked OK (straight back about 3"). We replaced most of the brake lines. The master was carefully bench bled. The lines were carefully bled by going sequentially from the master to each connection all the way to the caliper bleed fittings. When bleeding the lines, we made sure there were no contaminant (from the softened paint in the old master).

The end result was a slight improvement in braking efficiency but it's nowhere near right. The car it still does not stop on a dime (or even 2 bits!). The pedal bottoms out soft/mushy, comes up slightly when you pump the brakes, but no matter how hard we do all that, we arenowhere near locking-up the wheels (front or rear).

I AM AT A LOSS ON WHAT ELSE TO DO.



KONADOK

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Old 12-23-2005, 09:57 AM
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sounds like you still have air in the lines. If the M/C is below the level of the calipers / wheel cylinders, it is difficult at best to bleed them well. When i finally got mine to feel good and stop REAL well. i bled them is this manner. First, get the car up on jackstands. Crack all of the bleeder screws open & gravity bleed....overnight (i know, it wont bleed completely, this is the first step) next day, get yout hand held vacuum pump (you DO have one don't you ), start at the furthest bleeder, * operate the pump until you get clean fluid fron all 4 corners. This might take 2 or 3 rounds, but it will work

just my .02
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Old 12-23-2005, 10:36 AM
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The information contained in http://www.ecihotrodbrakes.com/brake_facts.html may be useful. Does the system have the proper residual pressure valves?

The system does sound as if there is air in it.

If the master cylinder is below the calipers / wheel cylinders I don't believe gravity bleeding will work. You may have to go the presure bleed route.
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Old 12-23-2005, 11:26 AM
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I agree with pressure bleeding.

Your rubber lines cannot be swelling that much.. so,

There are units out there that essentially are a pressure tank with a gallon of fluid in them that hook to the top of the master cylinder, and you put regulated compressor air pressure on top, bleed the wheel at such a rapid rate and volume that any air will have to be forced out .. duck.... here it comes.... rapidly.!!!!!! You will blow the entire line to the rear wheel full of fresh fluid in 2 seconds.

I have made an aux MC cover with an air hose fitting in it, and used a differential pressure gauge to apply the air above the fluid... but you must be careful... if you run the MC dry, you start all over.


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Old 12-23-2005, 02:00 PM
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Bad M/C. Its probably not new, but rebuilt, and GM master cylinders don't rebuild so well. They are cheap, porous castings that just suck. I have one in my wagon right now that is so porous it actually leaks THROUGH the casting. The whole outside of the casting is bone dry, but right on the bottom there is a little inclusion in the casting that seeps and leaks.

In my experience about one in ten rebuilt iron GM M/Cs is absolute junk. I've gone through three in the last year.

Make sure (like the others said) that there is no air in the lines, but it truly sounds like a bad M/C
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Old 12-24-2005, 12:05 AM
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"The pedal bottoms out soft/mushy, comes up slightly when you pump the brakes"

I wonder about a few things, having not seen the car.
-Do you have the correct metering valves in the system? Without them, you will not get a pedal, as there is too much fluid required to move the pistons all the way out.
-Is the Master Cylinder adjusted properly? Not enough throw will give you inadequet fluid flow.
-Are the calipers flexing? I have seen excesive caliper flex give a soft pedal that was taken for air. Check the mountings for the calipers.
-Are all the pistons staying out, or do you have one or more retracting due to a dry/sticking piston seal. This will definitly give a soft pedal.
-Are all the brake lines and hoses secured? If one or more moves during braking, then you will be using fluid pressure that should goto stopping the car, not flexing the lines.
-Good luck, please repost results.
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Old 12-24-2005, 12:48 AM
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New booster. Your old one has leak. Judging by what yo have siad thats all that can be left. i owuld check the bleed one more time then get a new booster.This has happened on more of my old cars than i can count. this exact thing occurs.
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Old 12-24-2005, 11:26 AM
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make shure that you have a 2 PSI residual valve on both lines, front and back and I assume you have a frame mounted M/C. then make shure all calipers are on right side up, or bleeders up, if there not and you got bleeders down, no matter how much you bleed them, you will never get it bleed, because if the air rises to the top, but the bleeder is not at the top, the air cannot escape.
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Old 12-24-2005, 01:32 PM
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Quote:
Originally Posted by streetrod27T
New booster. Your old one has leak. Judging by what yo have siad thats all that can be left. i owuld check the bleed one more time then get a new booster.This has happened on more of my old cars than i can count. this exact thing occurs.
Aloha streetrod27,

I installed a new booster (went from a 7" single chamber to a 7" double chamber sold with the master as a matching assembly).

Last edited by KONADOK; 12-24-2005 at 01:56 PM.
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Old 12-24-2005, 01:45 PM
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Quote:
Originally Posted by TechnTool
"The pedal bottoms out soft/mushy, comes up slightly when you pump the brakes"

I wonder about a few things, having not seen the car.
-Do you have the correct metering valves in the system? Without them, you will not get a pedal, as there is too much fluid required to move the pistons all the way out.

REPLY - We installed 2# residual valves on both the front and back lines and as near to the master as feasible. We also installed an adjustable proportioning valve on the rear line after the residual pressure valve.

-Is the Master Cylinder adjusted properly? Not enough throw will give you inadequet fluid flow.
REPLY - The activating rod from the booster was adjusted to start about 1/16" from the master cylinder piston.
QUESTION - Is there another adjustment?

-Are the calipers flexing? I have seen excesive caliper flex give a soft pedal that was taken for air. Check the mountings for the calipers.
REPLY - The set up is rigid. No flexing of any of the brackets, lines or calipers.

-Are all the pistons staying out, or do you have one or more retracting due to a dry/sticking piston seal. This will definitly give a soft pedal.
QUESTION - How do I check that?

-Are all the brake lines and hoses secured? If one or more moves during braking, then you will be using fluid pressure that should goto stopping the car, not flexing the lines.
REPLY - The set up is rigid. No flexing of any of the brackets, lines or calipers.

-Good luck, please repost results.
REPLY - Thanks for the questions & suggestions. Keep the good feedback coming. With the help of folks like you, I WILL get to the bottom of this!

See response(s) above.

Last edited by KONADOK; 12-24-2005 at 01:55 PM.
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Old 12-24-2005, 01:52 PM
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Brakes that don't brake.

Quote:
Originally Posted by TechnTool
"The pedal bottoms out soft/mushy, comes up slightly when you pump the brakes"

I wonder about a few things, having not seen the car.
-Do you have the correct metering valves in the system? Without them, you will not get a pedal, as there is too much fluid required to move the pistons all the way out.
-Is the Master Cylinder adjusted properly? Not enough throw will give you inadequet fluid flow.
-Are the calipers flexing? I have seen excesive caliper flex give a soft pedal that was taken for air. Check the mountings for the calipers.
-Are all the pistons staying out, or do you have one or more retracting due to a dry/sticking piston seal. This will definitly give a soft pedal.
-Are all the brake lines and hoses secured? If one or more moves during braking, then you will be using fluid pressure that should goto stopping the car, not flexing the lines.
-Good luck, please repost results.
My apologies to "TechnTool" & "streetrod27". I inadvertently directed to "streetrod27" responses to "TechnTool" questions.
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Old 12-24-2005, 07:22 PM
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What calipers do you have?
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Old 12-24-2005, 09:27 PM
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Thinking out loud (?).

Let see
2 boosters
2 MC
new lines
pumped 20 gallons of fluid through it......
pedal comes up a little in a couple pumps...

1) still got air
2) 2 bad MC in a row...

I'd take the new MC back and try another. I had one that seemingly bench bled OK but would not work.
If it doesn't help, I'd make a pressure bleeder.
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Old 12-25-2005, 01:29 AM
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Quote:
Originally Posted by KONADOK
My apologies to "TechnTool" & "streetrod27". I inadvertently directed to "streetrod27" responses to "TechnTool" questions.

Not a problem here. let us know what it was when you find out.
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Old 01-09-2006, 12:25 AM
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Brakes That Don't Brake

I want to put closure to this thread because a couple of people had asked that I post the outcome and because it will probably make some folks gasp and others snicker at the stupidity of the whole thing. So here it goes....

The problem was traced to the master cylinder being powder coated along with the other undercarriage components. The folks that did this took the cover off before the coating so the inside of the master was coated. The brake fluid dissolved the coating. We found paint gunk in the piston and most of the lines. We replaced the master and replace all the lines. We bench bled the master and gravity/pedal pump bled the calipers. We now have brakes that brake OK. I am confident that they will work even better when we figure out a good way to "power bleed" the system.
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