Hey All,

recently purchased a '78 F100 (2WD, with vacuum brake booster, rear drums, front discs). The existing master cylinder had a lot of sediment build up in it and many other parts of the brake system needed to be replaced. I replaced the master cylinder with an original style cast iron master cylinder; however, the reservoir gasket doesn't seal well and brake fluid leaks out regularly from the gasket. I'm now looking at replacing it with a plastic reservoir with screw on cap type master.

Question is: How critical is certified compatibility? I'm having trouble finding a master cylinder (with the plastic reservoir) that is intended for a '78 F100. I figure as long as the push rod pushes the pistons it should be alright, or is that wrong? If I'm understanding things correctly I should be able to adjust the length of the push rod protruding from the booster as well. I know bore diameter will affect the fluid pressure coming out of the cylinder, so if I keep that dimension close to the existing cylinder I should be okay?

Any help/advice would be greatly appreciated.