i have no brakes on 1991 GMC Suburban 2500

i have can you tell if its your brake boost thats gone bad or your master cylinder

Asked by for the 1991 GMC Suburban 2500
If the brake servo unit fails the brake pedal goes hard and a lot of pedal effort is required to stop the car. If the master cylinder is bad the pedal may travel to the floor, brake master cylinder may not be able to compress the brake fluid to generate brake fluid pressure. You may also have a brake fluid leak but it sounds like the car is unsafe to drive.
To check the master cylinder I often put line clamps on the brake flex lines to see if the pedal improves or put blanking plugs in the metal lines on the master cylinder to prove the cylinder is bad.
For brake bleeding instructions Autozone have some popular workshop manuals for free on line once you register (for free) on their website or there is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You can get a one week subscription or year subscription very cheap. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual try
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i checked theres no leaks but its alot of air in the line i've blead the brakes but someone told me its a certain way you bleed the brakes with abs do you know oh and i dont drive it at all