If your 1971 F-250 has a spongy brake pedal or poor braking performance, the issue is *not* a master cylinder adjustment. The problem likely lies elsewhere in the braking system. Here are the more common causes and how to address them:
* Low brake fluid: Check the brake fluid level in the master cylinder reservoir. If it's low, you need to add DOT 3 brake fluid (check your owner's manual to be sure). A low level indicates a leak somewhere in the system, which needs to be found and repaired.
* Leaking wheel cylinders or calipers: Wheel cylinders (on drum brakes) or calipers (on disc brakes) can leak, reducing braking pressure. Inspect them carefully for leaks. This requires replacing the leaking component.
* Worn brake shoes or pads: Worn brake linings reduce braking effectiveness. Inspect the brake shoes or pads and replace them if they are worn down to the wear indicators.
* Leaking master cylinder: While you can't adjust a master cylinder, it *can* leak internally. This is a much more serious problem requiring master cylinder replacement. A spongy brake pedal, along with low fluid level despite no visible leaks elsewhere, is a strong indicator of this.
* Air in the brake lines: Air in the brake lines will make the pedal feel spongy. This requires bleeding the brakes – a process of pushing air out of the brake lines by opening bleeder valves at each wheel while an assistant pumps the brake pedal.
* Master cylinder pushrod adjustment (possible, but rare and usually not necessary): Some older vehicles *might* have a very slightly adjustable pushrod on the master cylinder, but this is usually factory-set and shouldn't need adjustment. If you're considering this, it's highly recommended you consult a repair manual specific to your truck's model. Messing with this improperly could cause further problems.
In short: Don't try to "adjust" the master cylinder. If you have brake problems, diagnose the root cause using the points above. Bleeding the brakes is a relatively simple DIY task if you're comfortable working on your vehicle, but replacing parts like wheel cylinders, calipers, or the master cylinder is best left to someone with experience unless you are very mechanically inclined. A spongy brake pedal is a serious safety concern; don't drive the truck until the problem is fixed.
