Join Date: Nov 2000
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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.