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**jopizz** · December 26, 2014, 08:55 PM

If you don't see any visible signs of a leak then most likely it is leaking into your booster. You can remove the two mounting bolts on the master cylinder and pull it out slightly from the booster. If you see any brake fluid at all on the rear of the master cylinder or on the push rod then that's your problem. You can replace the single master cylinder with a dual master cylinder from a late 60's Mustang if you want to go with a safer setup. You will just have to split the lines front and rear. The stock single master cylinder is rather easy to find.

John

**simplyconnected** · December 27, 2014, 01:18 AM

John is right. A booster will hold a lot of brake fluid before it fills up to the vacuum hose. Brake fluid is not oil-based, it's glycol based and it won't burn. If you see funny looking smoke that periodically comes out your exhaust, it's probably brake fluid.

For this to happen, your Master Cylinder's piston seal leaks and your booster seal leaks as well. When you get it fixed, you will notice a big difference in your braking. - Dave

**ramos291** · December 28, 2014, 02:40 PM

Thanks so much John.

**ramos291** · December 28, 2014, 02:43 PM

Thank you Dave for the response and great information.

**ramos291** · December 28, 2014, 07:59 PM

John, I have been looking on Rock Auto for a replacement master cylinder, so many to choose from and there is also the issue with those being for ft disk and rear drum. Does that matter because my bird is all drum at this point?

**jopizz** · December 28, 2014, 08:17 PM

You want a drum/drum master cylinder for power brakes. The Cardone numbers are 101488 or 131488. They are listed under 1968 Mustang. The only issue you may have is with the shock tower to firewall strut. You may have to raise it to clear the master cylinder top.

John

**ramos291** · December 29, 2014, 03:51 PM

Thank you for the Cardone numbers and warning about shock tower firewall issue.

**ramos291** · January 27, 2015, 06:46 PM

I have installed the new dual master cylinder, connected it to a proportioning valve, bleed brake line and so forth. However, when pressing on the brake peddle, engine running, the idle speed increases and becomes rough. let go of the peddle and idle smooths out. Also when testing the braking in short spurts on the drive the peddle seems hard and takes a good amount of effort to stop the car. The new master cylinder is for a 68 mustang and did not come with a new plunger rod so I used the old one I had with the old single master cylinder. The vacuum lines also seem ok and the brakes before the switch to a dual also seems good. I find it hard to think the brake booster would just go bad after the switch when it was ok before. The booster had a good amount of brake fluid in it and that is why I changed it out. I did suction out the fluid before installing the new master cylinder. Any thought, suggestions, prayers to help? I am lost on this.

**jopizz** · January 27, 2015, 06:57 PM

You obviously have a vacuum leak related to the brake booster or check valve. The master cylinder and prop valve have nothing to do with losing vacuum. It's possible that the rod is too short for the new master cylinder and you are forcing the booster to go past it's limits when you push on the pedal. That's the only thing I can think of.

John

**jopizz** · January 27, 2015, 07:25 PM

I believe the rod has an adjustment nut on it. You can try and make it longer to see if it helps.

John

**simplyconnected** · January 28, 2015, 07:33 AM

Which M/C did you buy? What is the bore diameter? This makes a big difference in pedal pressure.

The first red flag is, having brake fluid in the booster. That tells me your old M/C AND your booster both leaked. More specifically, your booster sucked all that brake fluid out of the M/C. Fluid should never have passed by the M/C's piston seal.

So the idea that your booster was ok is not valid. It leaked and filled with fluid. Having a hard brake pedal is a classic and common booster problem. I suggest you find a new one to suit your system.

The change to a dual M/C and a combination prop valve is a step in the right direction.
John is correct regarding your adjustments. There is a jack screw between the booster and master. With the bolts off, separating the booster and master, have someone step on the pedal and watch the adjustment screw come out of the booster. Follow your Shop Manual for proper setup. Basically, the adjustment screw needs to touch the piston when at rest. I'm anxious to hear your response. - Dave

**RustyNCa** · January 28, 2015, 11:27 AM

I agree with Dave, you can't just drain the brake fluid out of the booster and continue to use it. You need a new booster. The brake fluid in the booster kills the booster's diaphragm and you will have a problem with it not providing boost and also creating a vacuum leak.

**ramos291** · January 30, 2015, 06:19 PM

The dual M/C is from RockAuto.com for a 68 Mustang Part # 131488. I am at best learning my Mechanical skills as I go on this old car from what my dad taught me. the M/C was suggested by someone with more experience then I. I am sure the fluid the M/C took on could not of been good for it but the Car's braking "seemed" to be fine before the change. I think it would be a good idea to change it when I find one that will work.
This site is great and I have gotten good responses with different issues i have encountered. THANK YOU ALL. Your advise is appreciated

**jopizz** · January 30, 2015, 11:18 PM

That master cylinder is the closest dual master cylinder to the original single master cylinder. But anytime you fit parts that weren't made for your car there is always trial and error involved. As I mentioned earlier you will probably have to adjust the push rod or get a longer one. If you measure from the end of the master cylinder to where the rod contacts the piston on both the old and new master cylinder that will tell you how much of a difference there is. However I don't believe the problems you are having with the booster has anything to do with the master cylinder. What you are describing is typical of a leaking booster.

John

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