Okay, if there is no fluid loss, you replaced the master cylinder, and the booster is working correctly, have a helper apply the brakes while you watch the rubber brake hoses to see if they expand. It is a long shot, but nothing else makes sense.
Also, you asked the best way to check the brake booster. Here are the directions for testing from Alldata:
Brake Booster
1. Check the hydraulic brake system for leaks or insufficient fluid.
2. With the transmission in NEUTRAL, stop the engine and apply the parking brake control. Apply the brake pedal several times to exhaust all vacuum in the system.
3. Apply the brake pedal and hold it in the applied position. Start the engine. If the vacuum system is operating, the brake pedal tends to move downward under constant foot pressure. If no motion is felt, the brake booster system is not functioning. Continue with the following steps.
4. Remove the vacuum booster hose from the check valve connection. Manifold vacuum must be available at the check valve end of the vacuum booster hose with the engine at idle speed and the transmission in NEUTRAL. Connect the vacuum booster hose to the brake booster check valve and verify that the manifold vacuum is available to the brake booster.
5. Operate the engine a minimum of 10 seconds at fast idle. Stop the engine, and let the vehicle stand for 10 minutes. Then apply the brake pedal with approximately 89 N (20 lb) of force. The brake pedal feel should be the same as that noted with the engine operating. If the brake pedal feels hard or no downward movement is felt (indicating no power assist), install a new brake booster check valve and retest. If the concern is still present, install a new brake booster. If the brake pedal feels spongy, bleed the hydraulic system to remove air.
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One last thing, here are the directions for checking the check valve. You mentioned it was good, but I thought I would add this to make sure it is how you checked.
Check Valve
The function of the brake booster check valve is to allow manifold vacuum to enter the brake booster and prevent the escape of vacuum in case manifold vacuum is lost during sustained full throttle operation.
To test the function of the brake booster check valve:
Start and run the engine for at least 10 seconds.
Operate the brake pedal to check for power assist.
Disconnect the vacuum booster hose from the brake booster check valve. Do not remove the brake booster check valve from the brake booster.
There should be enough vacuum retained in the brake booster for at least one more power-assisted brake operation.
Let me know.
Joe
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Wednesday, March 10th, 2021 AT 10:59 AM
