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Old 05-20-2007, 05:37 PM
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Brake booster troubleshooting...

Help that anyone can provide is appreciated - this problem has been kicking my butt for 2 months now. I just completed my 47' Ford - it has a 4.6 DOHC Mod motor and 4R70W transmission.

I am using your typical under floor mount 8" booster and corvette style dual master cylinder from Speedway Motors. Wilwood front discs and Ford drums in the rear. Master cylinder has been bench bled, and all wheels have been bled and I have a good firm pedal with no power.

When I start the car and the booster kicks in, the first time I push on the brake pedal, it sticks half way down and the brakes drag . If I pull the pedal back up with my foot the brakes release. Push it and it sticks - almost like the booster is getting too much vacuum? Is that even possible?

I have replaced the booster twice with the same result - so it doesn't seem like a booster malfunction. I have replaced the master cylinder twice and it again tests out fine. Anyone have any ideas?...

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Old 05-21-2007, 04:38 AM
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Have you checked the booster rod length that engages the Master cylinder. If it's too short it could cause the Booster diaphram to travel further forward than necessary. This could cause it to stick forward.

The rods are usually adjustable for a small amount. Some master cylinders have quite a deep recess in the piston. I had to make one up for our setup. It was that many years ago (over 20 years now) I forget what clearance I use but it was only small like 1/16" or 1/8" free travel between Rod and Master Piston. There are specifications for the length of these rods, but when you mix and match Booster/Master setups, you can get lost real quick.

Hope this helps.......Barry.
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Old 05-21-2007, 04:56 AM
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booster/Mc rod
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Old 05-21-2007, 12:17 PM
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Is a return spring bad?

I thought it might be the booster pushrod length too Barry - but I checked that and it is correct for the M/C. I talked to a guy at Speedway (where the booster and M/C came from) and he said I need to use a return spring on the pedal. They say they have had problems with under floor mounts and the sensitivity of power breaks where as the car drives down the road, the vibration of the pedal makes it go down and down until the brakes are applied by themselves.

That still sounds weird to me - can you think of any reason that a return spring would be a bad thing?
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Old 05-21-2007, 02:51 PM
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OK, that's good that you have checked the pushrod length and it's correct. Yes I guess that many under floor type pedal systems could cause a problem. They are often not balanced by virtue of design (like our under floor system I'm building in our 27 T Roadster) as compared to under dash swing pedals.

Many cars used return springs on the brake pedal up to the mid-late 60's, so there would be no problem fitting one. The spring only needs to be strong enough to return the pedal to its stop. It's just fitting it in a suitable position and making it look neat.

Cheers.........Barry.
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Old 05-21-2007, 08:29 PM
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You might need a assist-helper spring.They make it difficult to apply the very first inch,but then they assist the down stroke continuation.The inner cone spring could also be weak.Check by running a vacuum gauge and then create a hole to slightly leak a bit and diminish the booster effectiveness.That will confirm or not.A Y pipe picking up no vacuum air from the air filter could diminish the boost.But it is a weird problem.
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Old 06-19-2007, 01:19 PM
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did you get it taken care of? I have the same issue going on right now, same deal ,MC/Booster rod is correct
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Old 06-19-2007, 09:15 PM
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How I solved it...

I put on a return spring that I got from the hardware store. I ran it from the pivot point of the boster pushrod to the frame and it works great. I talked to a guy at Speedway Motors and he said it is not an uncommon problem and the helper spring is a perfectly acceptable solution.
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Old 06-19-2007, 11:34 PM
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Excellent !.
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Old 06-20-2007, 08:07 AM
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I did that already same problem I think i'll replace the booster or go manual disc,actually I could use the room under the hood anyway
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Old 11-16-2009, 01:02 PM
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sticky brakes

did you check the rubber flex hoses conecting to your calipers? if they are old sometimes they will collapse when you aply preasure to the brakes resulting in the brakes sticking or dragging, eventualy they release the presure until the next time you hit the brakes.
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