93 toyota previa brakes get hard after driving for about 25 minutes.

Brake booster vacuum 'check valve' or booster itself is not functioning properly. That's my guess.

Brakes get hard? Do you mean pedal gets hard to push.

FLUSH brake system , sounds like you got moisture in the fluid and as it heats up it expands , causing the brakes to engage by themselves. Been about 15 years since I've seen that , but it's possible.