Odd initial suggestion: check your vacuum lines and fluid level/ condition in the reservoir.
Are you experiencing any other drivability issues? Maybe you could do a bleed on your brakes hydraulic fluid to check for general condition. Make sure that all of the calipers are moving freely and that any maintenance on that system is up to date. Only because it is a system full of wearable components, I recommend an awful lot of things before replacing other components.
Thankfully valves can be tested for proper operation quite easily and if you are to need to replace one, then you can be certain it has failed.
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