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Soft Brakes
#1
Finally got the new rebuilt steering box put in, New rebuilt brake booster, and new master cylinder. Bench bleed MC, bleed all four wheels. Took the car out for a little test run. Steering is great, brakes not so much. Pedal real soft, goes about 3 to 4 inches then the brakes come on like gang busters. Pump it up does get firmer. What could it be? No air in lines that I could see when bleeding. Have rear disc, new stainless lines to the front calipers. Everything is New or rebuilt. Except distribution block, could that be it?

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#2
couple of things.

you could still have air trapped in the system possibly the prop valve. you could try a vacuum bleed and pull the fluid through the system or use a long hose in a cup and just keep pumping fluid through it till the bubble comes out.

was the master converted to disc/disc? was the prop valve converted to disc/disc? or everything is stock disc/drum?

if you have stock brake equipment you may need to swap the prop valve for a late model with a disc disc configuration. or install a adjustable prop valve. the rear brakes are suppose to come on before the front.

to me i think you still have trapped air in the system. but a incorrectly set prop valve would cause the same issue.

in a not stock situation i've heard the push rod for the brake pedal from the booster can be too short and cause piston on the master to not move in far enough that would cause a similar effect but you would still not have full power to the brakes. it would engage very slow then lock up with the pedal on the floor. you can adjust the push rod and make it longer, but if you are using the stock master cyl this is not the issue, it would be the prop valve or air in the lines.

you could have air in the lines and not know it, i had a problem like that and i bought a vacuum bleeder kit and blew like 2 bottles through my system and finally a large bubble came out.
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#3
72Hcode, I used a vacum bleeder. The back brakes seem to be getting fluid with the vac bleeder, front at first then nothing. I just did the front the old way, have someone pump as I worked the bleeder. Seemed to be all fluid coming out.
Last year I was running a SVO master with the 4 dics set up, worked, but brake pedal got real hard. The booster was 41 years old, so I thought I could use a new one. Then someone on-line said the old master would work, got a new one from advance auto. They said 1-inch bore should handle the back disc with just a single piston in each. I went this route and got new lines form MC to distribution block, hated how I had the lines for the SVO MC. Yes the distribution block is the original.
I just read that about the booster rod, I measured. Seem long enough, but now I think it might not be.
Thanks for the help.

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#4
Check the flexible brake hoses. If they are original they may have gotten soft with time. You may need a residual pressure valve in the rear brakes. Disc brakes usually keep a couple of lbs pressure on the caliper to prevent the piston from retracting too far.

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

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#5
Also be sure you have your calipers on the correct side. The bleeder valves should point just about directly horizontal. If they point slightly vertical, they are on the wrong side and you have an air pocket right there, even though it looks like no air came out while bleeding
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#6
The clearance of the brake push rod must be set correctly. There has to be enough clearance so that the master cylinder piston retracts fully, so the bore is filled with brake fluid, but not so much that you don't push the piston far enough forward.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#7
Talked with Roy today and he reminded me that he went through the very same as you. Said it turned out to be the emergency brake adjustment of all things. You may want to give it a try.

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Just cruising along minding our own business when BAM!!! The LAWS show up.
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#8
I adjusted the rod out of the booster some more. Seemed to help, then found the brand New MC was leaking out the back dripping down on the booster.
This thing is New not rebuilt. This could be some of my problem. It is going back to advance tonight or tomorrow. Should I by New or rebuilt? Seems like it is a 50/50 shot either way.
Marks73 I try to look at the e-brake also.
Thanks.

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#9
coupe351w;124962 Wrote:I adjusted the rod out of the booster some more. Seemed to help, then found the brand New MC was leaking out the back dripping down on the booster.
This thing is New not rebuilt. This could be some of my problem. It is going back to advance tonight or tomorrow. Should I by New or rebuilt? Seems like it is a 50/50 shot either way.
Marks73 I try to look at the e-brake also.
Thanks.

I got a scott drake MC and works great! Less expensive than the advance one and looks like better quality. I don't trust advance to parts as critical as my brakes. However their voltage reg works great! Smile
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#10
Thanks Matrixx, I just might look into a Scott Drake MC. Get my monet back from advance.Sad

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