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post #1 of (permalink) Old 09-09-2002, 08:31 PM Thread Starter
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Best way to diagnose GM booster swap?

About a year ago I swapped in the GM booster. My brakes are stopping the jeep, but the pedal is really light, and I never get a good feel outta them. I'm tired of pumping them up.

I've adjusted the rear brakes several times. Bled them several times. Just can't seem to get the brakes to work right.

The pedal has a SUPER light feel to it. The brakes eventually stop the jeep but the pedal feels down right sh!tty.

I'm wanting to pull the master cylinder back off and play with the rod that goes between the booster and the master cylinder. I've looked at it before, and the rod from the pedal to the booster. Both seem to be the right length.

Stock axles AMC20, Dana30. Nothing special, other than the S10 booster, and Jeep power master cylinder.

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post #2 of (permalink) Old 09-10-2002, 06:17 AM
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Re: Best way to diagnose GM booster swap?

If I recall, When I was finding info on doing the booster swap I was told that the pedal would be softer than stock. Did you use the cj master cylinder? I kep the chevy master cylinder when I did mine and its not that bad.
post #3 of (permalink) Old 09-10-2002, 06:50 AM Thread Starter
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Re: Best way to diagnose GM booster swap?

My Jeep only had manual brakes. I had to switch to a CJ Power master cylinder just to get the brakes to work. The pedal is softer, but it goes to the floor (if I want it to). The pedal just doesn't seem to have a limit, even if I pump it up...still seems like it's moving toward the floor.
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post #4 of (permalink) Old 09-10-2002, 09:33 PM
 
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Re: Best way to diagnose GM booster swap?

Could it be the proportioning valve????
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post #5 of (permalink) Old 09-11-2002, 08:38 AM Thread Starter
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Re: Best way to diagnose GM booster swap?

UHHHHH.... don't know.... could it be??
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post #6 of (permalink) Old 09-11-2002, 10:19 AM
 
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Re: Best way to diagnose GM booster swap?

You know there is that little tool you use on the valve when bleeding, I've never used it, but I had the exact same problem with my manual brakes you are describing, since I have the power brakes now I'm going to get a tool or fabricate one and try it.
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post #7 of (permalink) Old 09-11-2002, 11:08 AM
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Re: Best way to diagnose GM booster swap?

If you vacuum bleed the system at the wheels, you don't need the special tool for the prop. valve.
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post #8 of (permalink) Old 09-11-2002, 03:50 PM
 
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Re: Best way to diagnose GM booster swap?

I have the S10 booster and stock master cylinder and the brakes are not spongy. The system generates enough pressure that I am sure I can feel the hoses flex when I really press on it, but only a light touch is needed to stop, and hardly an effort to lock up all 4 33's on pavement. Must be a malfunction of some sort, so keep trying. The rod between the m/c and booster should have just a touch of play in it.

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post #9 of (permalink) Old 09-11-2002, 04:56 PM Thread Starter
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Re: Best way to diagnose GM booster swap?

Hey Tim, did you make ANY adjustment in the rod between the booster and master cylinder? I was wanting to leave it alone, and just mess with the rod that connects to the pedal. Or maybe get me an adjustable rod, and see if that helps.

When you say STOCK master cylinder, do you mean stock POWER brake master cylinder. Or stock MANUAL brake master cylinder?

My brakes are working, and I the pedal is not hard to push. I just expect it to STOP moving and it doesn't. But I'd hate to HAVE to stop. Always questioning the brakes is driving me nutty.

Thanks for the replies.
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post #10 of (permalink) Old 09-11-2002, 05:15 PM
 
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Re: Best way to diagnose GM booster swap?

I am using the stock power brake master cylinder. I was told that when switching to the s10 booster the stock rod between the booster and master cylinder would need lengthening. My brake man gave me a longer rod and told me to grind it down till there was about 1/16" clearance. Having failed to follow his instructions correctly and accurately, I left the rod slightly too long. One hot day as I was towing a trailer, the brakes slowly began to drag until it got so bad I had to loosen the bolts holding the master cylinder down to create clearance. That instantly solved the drag. I ground some more off the rod and very carefully felt to make sure it wasn't pressing in the master cylinder (hard to do with the brake lines connected). I heard another guy used a stock chevy master cylinder on that booster, I am guessing the one that went with it on the s10 and he had good results as well. But, anyways, I have a Dana 30 front end with disk brakes, all stock for an 84, and a Dana 44 rearend with 11" drums from a Scout II and this setup works fine. On the pirates board I got caught up into a debate over a guy advocating using a Ford F350 master cylinder, which has a larger diamter piston, and that it firmed up his brakes. He felt he needed more volume as he had swapped axles.

I've heard there is a residual valve on the drums to keep them from fully retracting. I don't know what it looks like, and there may be one on the disks as well. It maintains a few pounds of pressure in the line to the cylinder or caliper to prevent the shoes or discs from backing off and creating too much clearance, and thus demand for further pumping of the brake due to more volume. Some master cylinders have it as just an insert in the machined opening for the brake lines, and guys have taken them out for some reason.

As litte test might be to tighten your rear drums up solid tight with the adjusters and see if that makes any difference. Of course you can't drive like that, but it might shed some light, as if it makes no appreciable differnce, then it's nothing to do with the rear shoes and their clearance.
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