Install a vacuum gauge directly to the modulator vacuum hose, with the engine running there should be a steady reading of manifold vacuum present 14” to 22”HG. A quick opening and closing of the throttle should drop the vacuum to below 5” if not fix the vacuum problem first. If you have the proper responses, then install a pressure gauge and read working pressure. With the vacuum hose removed from the modulator and the car running at idle in drive hot, the pressure should read between 70 and 90 psi. Higher then 120 psi you could have a stuck pressure regulator valve (valve body) or a stuck modulating valve behind the modulator.