Most Likely Culprit: Power Brake Booster Leak:
The power brake booster uses engine vacuum to assist in braking. A leak in the booster diaphragm will prevent it from building vacuum, resulting in a hard pedal and a hissing sound as the vacuum escapes. The leak can be internal (within the booster itself) or external (at the connections to the booster).
Other Possible Causes (Less Likely but Still Possible):
* Brake Master Cylinder Leak: A leak in the master cylinder (the reservoir where brake fluid is stored) will also lead to a hard pedal, but typically wouldn't produce a significant hissing sound unless the leak is large and under pressure. You'd also likely see brake fluid leakage.
* Vacuum Leaks in the Vacuum Line: The vacuum line connecting the intake manifold to the power brake booster could have a crack or leak. This is less likely to cause a hard pedal as extreme as a booster diaphragm failure, but a noticeable leak in the line *would* cause a hissing sound.
What to Do:
1. Safety First: With a hard brake pedal, your braking power is severely compromised. Do NOT drive the vehicle until the problem is diagnosed and fixed. This is a serious safety issue.
2. Listen Carefully for the Hiss: Try to pinpoint the location of the hissing sound. This will help narrow down the source of the leak. Is it coming from the booster itself, or from the vacuum line?
3. Visually Inspect: Carefully examine the power brake booster and the vacuum line connecting it to the intake manifold. Look for cracks, loose connections, or any signs of brake fluid leakage.
4. Check Brake Fluid Level: Check the brake fluid level in the master cylinder reservoir. A low level *could* indicate a leak, but it's not the primary reason for the hard pedal and hissing in your case.
5. Professional Diagnosis (Recommended): Given the safety implications, it's strongly recommended that you take your S10 Blazer to a qualified mechanic for diagnosis and repair. They have the tools and expertise to properly diagnose the problem and safely repair the brake system. Attempting a brake repair yourself without experience can be dangerous.
Important Note: Brake system repair requires specialized knowledge and tools. Improper repair can lead to brake failure and serious injury. Don't attempt major brake repairs unless you have extensive experience and the proper tools. A professional mechanic is the safest route.
