1989 Ford 1/2-ton master cylinder question - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-09-2012, 10:59 PM
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1989 Ford 1/2-ton master cylinder question

This '89 Ford power brakes master cylinder is in my '57 Chevy Suburban, with front discs and rear drums. When installing that master cylinder, I have noticed that there is something at the rear brake line connector, where you connect the rear brake line, something that could look like some sort of "restrictor", something that the front brake line connection does not have. What is this thing?
I am asking because my truck will soon have rear disc brakes and I am wondering if I should remove that "thing" to get the proper pressure to the rear discs, and proper balance in the system.
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Old 03-10-2012, 08:02 AM
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That is a residual pressure valve. I am not sure of the pressure setting, but assume it's about 10 psi.
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Old 03-10-2012, 09:36 AM
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So, what is the use of that valve? Is it a drum brake thing? And when switching to rear discs, should I leave it alone or remove it?
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Old 03-10-2012, 09:55 PM
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It holds pressure on the rear wheel cylinders to keep the shoes near the drums. You can't just take it out, the fitting it screws into uses an Oring seal. I believe you would have to disable the 10 psi valve, and install a 2 psi valve for disc brakes.
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Old 03-12-2012, 06:48 PM
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I have something like that (aftermarket) along the rear brake line, because the master cylinder is lower than the drums.
So, what should I do when I switch to rear discs? Remove the inline valve? That 10psi valve you mention, is it a Ford part or an aftermarket?
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Old 03-13-2012, 11:38 AM
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It's a Ford part. For disc brakes you need a 2 psi valve. so disable the Ford valve on the master cylinder, and replace the aftermarket 10 psi valve you have with a 2 psi valve.
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Old 03-13-2012, 07:53 PM
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Ok, thanks, great detail on the residual pressure valve, I would not have known disc brakes required a different one!
Now, with discs all around, I would not think I need a proportioning valve, right?
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Old 03-14-2012, 07:52 AM
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yes, you still need one, so the front brakes apply first.
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