Thread: 1932 ford disc brakes locking up

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I have a 32 ford truck that I have been working on. My dad did a disc brake conversion from speedway motors in 2008 it has a 2lb residual pressure valve in the front and a 10lb in the rear for the mid 70,s ford nine inch drums on the rear end. The Speedway motors kit came with a 1978 thru 1988 Chevy mid size car caliper that are apparently used for modified race cars. The issue is after I replaced the master cylinder due to a bad seal I drive for about five minutes both front calipers start to lock up the more I drive the worse they lock up. After it sits for about an hour they release.
I have replaced the new master cylinder twice and also the residual pressure valve. One of the calipers has also been replaced. The caliper pins are lubed pistons are not sticking kind of at a loss ????