Rear Wheel Brake Cylinder Question

Old 10-25-08, 07:13 PM
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Rear Wheel Brake Cylinder Question

1990 Crown Vic Station Wagon.
A shop recently installed new rear wheel brake cylinders. They have a 1" bore. My shop manual says they should be 15/16" bore, which I confirmed with Ford Motor Company. The shop says 1" is what the parts manufacturer supplied and said they should be.
Since that time, my rear brakes overheated twice and I am wondering if it has to do with the larger size wheel cylinders. The local Ford dealer says it doesn't make any difference if they are 1" or 15/16".
I checked Napa and Kragen online, they both say 1".
Can anybody shed some light on this discrepancy?
Old 10-25-08, 07:39 PM
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It doesn't make a significant difference, 15/16 or 1"(The application pressure of both bores are very close) as long as the configuration of the cylinder fits properly in the brake. Assuming the fit is correct, the overheating is due to something else.

Overheating is caused by brake drag and not application pressure. When you have an overheating incident, raise the problem wheel and turn it to see if there is significant brake drag. If so, crack the bleeder to see if that releases the drag on the brake. If it does you have something in the hydraulics of the brake that is holding pressure in the line.

If cracking the bleeder does nothing, the problem is in the brake itself - either in the adjustment (adjusting wheel at the bottom of the brake) or a possible emergency brake cable that's keeping the brake engaged.
Old 10-25-08, 08:48 PM
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there are many cars that had different wheel cyl bore sizes in the same year. The fact this is a wagon would be relevent to the size as well.
a quick look at Autozone, while it did not provide the bore diameter, it did list a 10" and an 11" brake size. Those could very well have used different sized wheel cyl bores.

Bottom line; what was the diameter of the old cylinders. That is what size the new cylinders should be.

as to causing brake drag; not due to the cyl size. marbobj has that down well.

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