pinexcorp

UREKA!! Problem solved. Thank you guys. The push rod coming out of the booster was too short, adjusted an additional 1" and car now stops on a dime. It only took me one weeks and bunch of new gray hair to figure it out, and with the help of everyone here. Thank you. 1969-Mach1 had it right.. So I think the orinal problem had to do with incompatible booster/MC and good think to keep in mind when replacing MC to match and I think the difference is in the push rod. The price on the Mustang just went up. Not gonna burn it. Robert

Midlife.. You got it exactly right. Once I connect the break lines and system is bled brakes feel like a sponge and finally stop the car about 1" from the floor. In my original question I forgot to mention that the two front brake hoses were also replaced at the same time the MC was replaced. As mentioned the car running and braking perfectly.. almost to good to be true. Rear brake and other rubber hoses are ok . Front disk and rear...

Midlife.. You got it exactly right. Once I connect the break lines and system is bled brakes feel like a sponge and finally stop the car about 1" from the floor. In my original question I forgot to mention that the two front brake hoses were also replaced at the same time the MC was replaced. As mentioned the car running and braking perfectly.. almost to good to be true. Rear brake and other rubber hoses are ok . Front disk and rear shoes are well adjusted. A real puzzler I have been working on cars 50 yrs and never been puzzled by a problem.. My next step is to replace the booster just to take it out of the equation although I don't think that is my problem.

Yes Brian.. Thanks. The Car is all original with power disc brakes.. Correct pedal , correct booster. What I dont get is why if the pedal when pushed all the way to the floor pushes the boosted push rod like 5 inches.. and the MC max travel is 1 1/4" any adjustment on the stud only positions the pedal higher or lower... Maybe I still dont understand how a booster works but looking at cross sections it looks like is a straight connection to the MC with the diaphragm in between and vacuum on one side assist to help with the pushing..I am starting to think that the booster is only effective in a certain range.. so I need to adjust the push rod to see it makes any difference.. Maybe an incorrect booster was installed and only worked with the MC I removed.. I think that makes sense... My Mustang is a 1969 Mach I original S code 390 and C6 trans. Originally Raven black.

I am totally dumbfounded. Four days working on this issue and now nothing makes sense. No matter how much I bleed the brakes I can not get them to come up insted they go all the way to the floor. They do stop the car when it hits the pedal hits the floor. Removed the MC and checked the stud sticking out of the booster and measures about 1" Next I suspect the booster was bad so I placed a 3/4 inch wood board and braced to test out how much force the stud was pushing out with.. pressed the pedal and force was strong enough to actually break the board and stick out an additional 4-5 inches? Bench testing the MC and pushing on the piston the action of piston fully depresses is only like 1 1/4 " so the booster is working... or so I think. Next I installed the MC back on the booster.. filled with fluid.. bled the MC and plugged the ports tight. With engine and vacuum the brake pedal was so hard barely moved. This tells me the MC is developing pressure and booster is working.. Next I reconnect the brake lines and bleed system and bleed and bleed going thru about a quart of brake fluid... Checking for leaks. Pedal still goes to the floor.. same thing. Nothing makes sense where is the pressure going? Anybody want to buy a mustang before I burn it?

Thanks Brian. Yes I think the MC is the focus. I had no Idea the push rod out of the booster had adjustment. I will check lengths.

Core diameter of the MC I have now is 1"

Thanks for your comments. Unfortunatelly I returned the MC core that was on the car and no longer available for me to see if any different from what I got now. My mustang is original power brakes with disc brakes in front and was running great brakes were good until I noticed my MC leaking at the seat and possibly into the booster. I dont think the booster has been damaged. So I set out to replace and have gotten units from Oreillys and autozone and both do the same where the pedal goes all the way to the floor before breaks. I have read and seen videos where they describe two types of booster on the 69 a Bendix and an midland.. I believe the booster in my car is a bendix since the vacuum port is on the side and not infront of the MC... Oreillys had a call out for a MC for Bendix which is not available anymore and so I am now wondering if there is a physical difference in the two MC and I need to find one. Maybe my old MC was a Bendix?? I think one of the lessons learned here is if you but a part for which you need to return the core dont return until you install the new part and all is well. kind of lost know as to not sure how to proceed.

It would be nice to find out how this issue was resolved as I am having exactly the same problem. I have now replaced the master cylinder twice and have bled the line 5 times with same result. Pedal feel awful and goes all the way to the floor before stopping.