Have you made a change of the master cylinder or done some other type of modification to the brake system? It should not be necessary to add check valves to an all stock system since there's already one built into the master cylinder. The only purpose of a check valve is to hold a little pressure in the brake lines to prevent air from being sucked past the wheel cylinder pistons from temperature changes, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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