Test tools for autos

The vacuum lines on a 1987 Chevy S10 Blazer are responsible for routing vacuum to various components such as the brake booster, EGR valve, and other systems.
A vacuum line diagram for your specific make and model of Chevy S10 Blazer can be found in the vehicle's service manual or in some cases, it can be found under the hood on a decal or sticker.
If the vacuum lines are loose, cracked, or damaged, it can cause various issues such as rough idle, stalling, poor performance, or even a failed emissions test.
It's best to consult the service manual, a professional mechanic or the dealer for proper diagnosis and repair of vacuum lines issues. These lines are crucial for the proper operation of many systems in the vehicle, so it's important to fix them as soon as possible if they are found to be damaged.

Only a professional can do testing up to the
PCM ,to eliminate all other items & wiring issues
You could if you have wiring diagrams for your vehicle
& know how to use a volt meter & not ground out the pcm
while doing your testing
They don't fail often,so I would be testing the entire
engine compartment & all systems

WEll, something is causing the issue.... First, fully charge the battery, then disconnect the negative and put a volt meter or test light in series (between the terminal and the cable). Disable interior lights, and test for draw of current.
Essentially, if the light lights, too much current is being drawn out of the battery. So, you disconnect fuses, and other items until the light goes out. This should help you find the offending component.

IF no light lighting, perhaps the battery itself has issues. Leave it disconnected overnight, and see if it is discharged...If yes, the battery would be suspect.

There are diodes in the alternator that fail and can cause voltage to be drained. Some tests to the alternator fail to reveal the fault. the best test is an Scope, but you can isolate diodes and test with continuity tester.

The amount of power required to start your engine is significantly greater than what it takes to play the radio or burn the headlights, so those items working don't necessarily mean the battery is adequate. Your description indicates the following:

1. Battery: an old or damaged battery can fail without notice.
   
2. Belt: the squealing belt indicates that it is loose or glazed and does not have the tension in order to turn components at their proper speeds. This could lead to the alternator not performing as it should.
3. Alternator: loose connections, a failing component, or a loose belt can cause the alternator to perform poorly.

The above items can be checked by a qualified mechanic in a matter of minutes or less. Some part stores offer a free test of your charging system, whether the component is on the vehicle or you have removed the part and carried it in for testing.

There are a great number of items that can fail and give you that problem. Likely the two most common are the crankshaft sensor and fuel pump.
What you need to do though is to determine if you have spark (hold an ignition wire near the block after sticking a screw or other metal object into the plug connector and have someone crank it over...you should have a decent blue spark that will jump at least 1/4" gap) If you don't , you will need to check the ignition system components. To check pump, turn the key to "on" while someone listens at the tank) If silent, check the pump relay in the poser distribution center and the pump itself for power.
To do any testing, you will need a decent manual like haynes or chiltons (about $20 in most larger parts stores) and a good digital multi-meter.
Testing components is very important because parts are expensive and many are not easy to replace. Testing eliminates changing unnecessary parts. No decent shop will replace parts without testing...neither should you!!!
Good Luck