Brake pedal briefly gets hard

Oh a Riviera! I love those cars!

Anyways, that problem is usually caused by a vacuum leak somewhere near the power brake booster. The booster has the job of pulling vacuum pressure from the engine intake manifold and using it to help you push the pedal down. When there is a leak, you have to do all of the work yourself, which makes it feel like you are trying to stop a freight train.

Vacuum lines, which are made of rubber, start to dry out and leak -or- flatten out under pressure when they get old. Time and heat cause this to happen.

The booster is located under the hood, by the firewall, driver's side of the vehicle, behind the master cylinder. The master cylinder is actually bolted to it. There is a sizable vacuum hose on it going to the intake manifold. Inspect it and all other vacuum lines. Replace as necessary. Here is a general guide on checking vacuum lines: https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If you do not find a problem there, then the issue is either with the booster itself, or the check valve (the part that the vacuum hose is connected to). You can simply pull the check valve off and try blowing through it and then sucking back in. It should only allow for airflow in one direction. If it allows for flow in both directions, then replace it. It should not be more than $15.00 at the local parts store (I think). If that is not the problem, then you probably have to replace the booster itself. We will get into test and replacing it if we need to.

Actually I just thought of this too. It could, possibly, be something going on with your Anti-lock brake system. Something might be triggering it to push back. This could happen if one or more of the wheel speed sensors is reporting inaccurate data (maybe it shows that the wheel isn't moving) and the ABS module, thinking that that wheel is locked and skidding, tries to push back, relieving brake pressure at that wheel, and thereby trying to unlock it. You might occasionally notice the "ABS" and "brake" lights on the dashboard coming and going while you are driving (after the initial startup test). If you do see either of those lights coming on, then you should get the car scanned at any shop with a scanner capable of communicating with your car's body control module. The local parts stores usually do not have a scanner capable of that, so you will have to find a shop and pay for them to scan it. Be sure and get the actual code numbers, and not just their descriptions.

Let me know and we'll go from there.

Oh well. So much for the simple cheap fix. You are going to have to get it scanned. Anything else I can suggest is purely a guess. If you insist on not getting it scanned, then start checking your grounds for your ABS module weak grounds can wreak havoc on them. The module should be located on the driver's side of the car, under the hood, mounted to the strut tower. I personally would take it off and get a wire brush or something similar and really attack it. Let us know what you figure out and let us know