Truck is a 1979 GMC C7000 with a single axle on back with duals. Engine is a 366. Truck has a booster behind the drivers side front wheel. Truck hauls short of 400 bushel. I think my master brake cylinder is leaking some. Looks as if the firewall is wet inside and out and the master brake cylinder does loose oil over time. Searching I find a part number 10-57577 with a 1 3/4 inch bore and a 10-57579 with a 1 1/2 inch bore. Anyone know if they are basically the same. Could I use a 1 3/4 inch bore even if the truck calls for 1 1/2 inch bore. Takes two pumps of the brake pedal to get good brakes. Maybe the 1 3/4 inch bore would help to fix that problem. Thanks.