Gary Lewis Posted June 21, 2018 Share Posted June 21, 2018 IIRC the 150 M/C has smaller bore than the 250/350 master. Increasing the bore displaces more fluid per pedal stroke but also increases the pedal effort for a given line pressure. Bigger trucks have larger caliper pistons that need more fluid volume and achieve a correct ratio with the bigger master cylinder. Jim - With the larger booster does the 250/350 master/booster combo wind up giving the same pressure for the same pedal force? Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted June 21, 2018 Share Posted June 21, 2018 Jim - With the larger booster does the 250/350 master/booster combo wind up giving the same pressure for the same pedal force? Gary, I don't know the details of the boosters, just that the bores of the master cylinder are different. 15/16" v 1 1/8" I think??? The mechanics of fluid displacement in a closed hydraulic system are simple ratios. TIL that the pedals have a different pivot (changing the effort applied to the M/C by the operator) Prior I only knew of this difference with the Hydroboost pedal. If Ford changed the bolt pattern and the line fittings of the master it was to prevent the wrong one from being installed and likely causing injury. Link to comment Share on other sites More sharing options...
Gary Lewis Posted June 21, 2018 Share Posted June 21, 2018 Gary, I don't know the details of the boosters, just that the bores of the master cylinder are different. 15/16" v 1 1/8" I think??? The mechanics of fluid displacement in a closed hydraulic system are simple ratios. TIL that the pedals have a different pivot (changing the effort applied to the M/C by the operator) Prior I only knew of this difference with the Hydroboost pedal. If Ford changed the bolt pattern and the line fittings of the master it was to prevent the wrong one from being installed and likely causing injury. I understand the physics involved with the bore sizes of the master and slaves as well as the leverage change induced by moving the pivot point. But I don't understand the differences between the F150 and F250/350 boosters. However, they are obviously matched to the master cylinder and should be kept together. And, you have a good point about why Ford changed the bolt pattern and fittings. Which begs the question "Why attempt this change?", which brings us back to the questions of "Has the booster failed?" and "Why did it fail in just 2 years?". I found this explanation of how the boosters work. And, within it says: One quick test for leakage, is to turn the engine off and press the brake pedal. If the pedal still has one or two assisted applications before getting hard to press, likely no leak exists. And then it offers this regarding why boosters fail: When brake boosters fail the reason is often outside the booster itself. For instance a bad master cylinder leaks fluid into the booster. Brake fluid will cause deterioration of the diaphragm resulting in failure. If misdiagnosed, the failed booster can provide a vacuum to the rear of a replacement master cylinder and quickly ruin it, repeating the cycle. Link to comment Share on other sites More sharing options...
Pebcak Posted June 21, 2018 Author Share Posted June 21, 2018 I understand the physics involved with the bore sizes of the master and slaves as well as the leverage change induced by moving the pivot point. But I don't understand the differences between the F150 and F250/350 boosters. However, they are obviously matched to the master cylinder and should be kept together. And, you have a good point about why Ford changed the bolt pattern and fittings. Which begs the question "Why attempt this change?", which brings us back to the questions of "Has the booster failed?" and "Why did it fail in just 2 years?". I found this explanation of how the boosters work. And, within it says: One quick test for leakage, is to turn the engine off and press the brake pedal. If the pedal still has one or two assisted applications before getting hard to press, likely no leak exists. And then it offers this regarding why boosters fail: When brake boosters fail the reason is often outside the booster itself. For instance a bad master cylinder leaks fluid into the booster. Brake fluid will cause deterioration of the diaphragm resulting in failure. If misdiagnosed, the failed booster can provide a vacuum to the rear of a replacement master cylinder and quickly ruin it, repeating the cycle. As usual, all of you bring up good points. This is why Google is a dangerous thing! I've checked everything along the engine bay for air leaks, discoloration and missing fluid. I haven't seen anything. When I popped off the top to the Master Cylinder it's still full and fluid is still clear"ish". About 6 months ago I had to top off the front section a tad, but that's it. It's all been stable and sturdy since I swapped the Master Cylinder and Booster a couple years ago at the same time. I will stick with my original plan (Before Google) of replacing with the same booster. While doing so I'm going to do a good look at all the parts and connections in case I missed something. Link to comment Share on other sites More sharing options...
Gary Lewis Posted June 21, 2018 Share Posted June 21, 2018 As usual, all of you bring up good points. This is why Google is a dangerous thing! I've checked everything along the engine bay for air leaks, discoloration and missing fluid. I haven't seen anything. When I popped off the top to the Master Cylinder it's still full and fluid is still clear"ish". About 6 months ago I had to top off the front section a tad, but that's it. It's all been stable and sturdy since I swapped the Master Cylinder and Booster a couple years ago at the same time. I will stick with my original plan (Before Google) of replacing with the same booster. While doing so I'm going to do a good look at all the parts and connections in case I missed something. I don't think that missing fluid in the front section will find its way into the booster. But it probably doesn't take much fluid leaking into the booster to kill the diaphragm, and you might not notice the loss from the rear section. So check the booster to see if it has fluid in it when you replace it. Maybe you just got a bad booster. That seems to happen more and more often these days with new parts. Link to comment Share on other sites More sharing options...
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