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I have a 1967 mustang and I have been working on the brake system. There are manual drum brakes all around. There is a problem with the brass valve located by the master cylinder. I heard that the rod inside the valve is supposed to float and move easily, but mine is stuck. Is there some way to fix this and test it? If not, should I try to repair it or buy a new one?
 

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If the internal spool is frozen in the pressure differential valve, you can try getting it out using compressed air into one of the ports. Remove the end cap and nylon switch first.

I didn't have any luck getting mine out and usually when this happens the corrosion has destroyed the finely machined surface on the spool. The valve then needs to go into the trash.

If you wanted to test one, do it while it is installed. Open up one of the bleeder screws to create a fluid leak. The spool is supposed to shift when pressure is applied to the pedal. The uneven brake system pressure should shift the spool and illuminate the "Brake" indicator lamp on the dash. (Don't lift up on the pedal until the bleeder screw gets closed. It will suck in air.)

You reset it by closing the bleeder and then applying firm brake pedal pressure. It is reset when the light goes out on the dash.
 
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