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Unread 11-11-2010   #1 (permalink)
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Question 1970 Mustang-How hard is it to change a master cylinder

The title says it all, I've got to replace the master cylinder on my 70' coupe. It has power disc brakes and drums on the rear (i did make sure i bought the correct MC) Is this alot more complicated that it seems?
Also, I'm not exactly sure how to bleed the master cylinder.
any help would be appreciated!
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Unread 11-11-2010   #2 (permalink)
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William, Its really not that difficult at all. Just be careful when you disconnect the (2) brake lines connected to the old MC on the care so you dont wind up having to replace those as well. Personally, i would but you dont have to.

Make sure you use a specific brake line wrench when loosening / tightening the brake lines so you dont round off the brass hex of the fitting. The look just like a box end wrench with a slot cut in one side so you can slip it over the brake line.

Your basically just going to swap out the MC's Thats pretty easy to do, 2-bolts and 2-brake line fittings.


You can bench bleed your new master cylinder before you install it. It will save you a lot of trouble and minimize the brake bleeding effort you will have to perform to your brakes once you do the exchange.

Go down to any harbor freight and buy the bench bleeding kit for about $20 - $25.

Once you have installed the new MC, you will need to bleed all foru brakes at the wheels cylinders.
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Unread 11-11-2010   #3 (permalink)
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Default 1970 Mustang-How hard is it to change a master cylinder

Hello Pockets,
Good to see you are checking things out before starting.
This is a wise choice.
Basically like RF Mustang says - -it is pretty darn simple.

This will not take long to do

I would like to add a few little thoughts.

1. - Before starting - spray all the nuts with PB Blaster for a day before starting.
This will make the job much easier.
2. - As RF points out - -use a tubing wrench do loosen the nuts on the lines.

3. - Sometimes when you get a new master it will include 2 plastic fittings and short pieces of hose.

To bench bleed the master - - do the following.

a. - Put the master in a vice to hold it secure. I grab the master by the "ears" where the bolts go. DO NOT put force on the bowls - they can distort.
b. - -attach a fitting to both outlet holes on the master. the fitting will have a little nipple that you attach the rubber hose to.
c. - Run the little rubber hose back up to the master bowl.
d. - Take a socket extension and push on the little piston
inside the end of the master. It is where the rod will attach.
e. - Pump the extension - several times. Watch the hoses. As the air in the master
is forced out - -it will bubble in the bowl.

f. - When you don't see bubbles in the master - - you are done bleeding.

SO as I say - -some masters will come with the little fittings and hoses. If not you can get a kit from most parts stores.

Bleed the master - - then loosen the 2 lines. Then remove the 2 bolts holding it to the booster. Remove - -and re-install.

remember - -when you place the rod into the master - -it will stay in there. It is made to not come apart.

This is a simple job that I know you will do well.

Of course after the install - bleed out the brakes at each corner.

I start pass rear - then driver rear - -then pass front - - driver front.

If unsure how to bleed - -just let us know

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Unread 11-11-2010   #4 (permalink)
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Wow awesome thank you for the prompt responses! I will definitely be using tubing wrenches and I have thought ahead and bought new lines as well. I just wanted to make sure there wasn't any sort of black magic trick to adjust or line up the push rod inside the master cylinder.
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Unread 11-11-2010   #5 (permalink)
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Well I bled the new master cylinder according to the instructions and put it on. And there was no change at all
My problem is my brake pedal goes all the way to the floor, it's so bad that if I'm driving the car when I stop with it in gear (auto trans) I have to hold the pedal down, hard, or the car will coast foreword. I've bled the brakes 6 times HELP ME!!
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Unread 11-11-2010   #6 (permalink)
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Default 1970 Mustang-How hard is it to change a master cylinder

Hello Pockets,
I felt badly because I had to run out and DID NOT mention unhooking the
master cylinder rod from the brake pedal. That is not too bad either but the
switch can be a little tough to get to.

My NEXT advice - - Do NOt drive the car until we figure out what is going on.

When you did the bleeding where you able to get fluid at all 4 wheels?

What I think I would check next is to jack up the car and support the rear end.
Get the tires off the ground but be certain the car is supported safely.

Start the car - -let her idle - - put the car into gear so the rear wheels are turing
off the ground. Step on the brakes and see if the tires stop spinning.

You want to check this with the car supported and NOt while driving.

It kinda sounds as though the brakes are not adjusted or are not working.

When you pumped the brakes to bleed - -did you get a hard pedal?

Just to review - -when you bleed the brakes - -someone pumps the pedal a few times and then holds pressure on the pedal. A second person (you) opens the bleeder valve on the cylinder as the person inside holds the pedal steady which should drop to the floor. Once the pedal hits the floor - - that person holds the pedal and you close the bleeder valve. Once closed - - the person inside the car pumps the pedal and you repeat the process. They put pressure on the system - -you open the valve - -the pedal drops - -you close the valve - they release the pedal. If not done right - -you are pulling air back into the system.

The next step would be to pull the rear drums and see if the shoes look good and all the hardware is attached. You may need new shoes perhaps in the rear or perhaps they are way out of adjustment.

From here - -I would lower the rear and jack and support the front.
DO NOT trust a regular jack. Once jacked - us jackstands.

Now you spin a tire by hand and have someone hit the brake. See if the tire stops rotating. Do the same fro the other front.

It is possible that you have a bad caliper. It could be sticking or not even working.

Gotta determine if all the parts are working and adjusted correct. If the calipers seem to be working - -take a look at the pads. Perhaps they are worn.

PLEASE let us know - we can then think of other ideas.

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Unread 11-11-2010   #7 (permalink)
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Just for reference my car has factory power disc front and drum rear brakes
The only thing that hasn't been replaced as of today is the brake booster and the pedal. no i'm not kidding.
i have new calipers, rotors, drums, shoes, all the hard lines as well as all of the rubber lines have been replaced including all of the hardware. (new springs for the drums and all) I used a bendix brake manual to assemble everything and i went over it twice with my grandfather to be sure it was correct.

there are No leaks, i have wiped everything down and checked the entire system for leaks.

I get fluid at all four corners and it shoots out with plenty of force

I follow your process exactly when bleeding the brakes

I have adjusted the drums to where there is the correct amount of drag as far as i understand there is no way to adjust the disc brakes..

When i pump the brakes with the engine off there is some resistance but it never "pumps up" significantly

Soo as near as i can figure SOMETHING is not bled properly... but WHAT!?
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Unread 11-12-2010   #8 (permalink)
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Default 1970 Mustang-How hard is it to change a master cylinder

Hello Pockets,
This is BAD. We gotta figure it out.

When you bled the brakes did you do the rotation several times?
Also where you abel to bleed the master?

It sounds to me as though something is not right in the rear.

I assumed you had power disc and drums. A good set up that should work.

Pockets, it is possible you still have air in the system but I am thinking it is something
else.
When you installed the rear drums are you certain you put the shoes in the correct location. The larger of the 2 shoes goes toward the rear of the car.

You are right - -there is no adjustment on the calipers.

I know this has to be getting frustrating but hang in there - - we will help.
Any chance of taking a picture of the rear brake so we can look at a few things.

This is real odd - when I think about it - -you say the car will move if you don't hold hard force on the pedal. The front brakes should hold the car.
ODD for sure.

If you can't post a picture - -I would try to bleed the system again. When you bleed each wheel - -do it about 4 times per wheel following the rotation.

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Unread 11-12-2010   #9 (permalink)
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Hey William, Print dad is correct there is still air in your system. Can you explain in what order did you bleed the brakes and how you bled them. (Right to left front to back.)???? Vary simple we will get you going right away. First off have two people one pumps the brakes while the other opens the bleeder screws. Always start at the farthest wheel from the drivers seat, the pass rear wheel pump 6 times and make sure the brake peddle comes all the way up and all the way down to the floor each time. OK start with pumping 6 times to the floor and hold it. Crack the rear pass side bleeder screw open let all air out and then close it, get the person to do it again pump 6 times and hold the peddle to the floor and crack the bleeder screw open again keeping the peddle to the floor now close the bleeder screw. So now go to the rear drivers side and repeat the process then move to the front pass side and do the same and then finish off with the front drivers side and don't forget to fill the master when finished. WOW fingers getting tired.
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Unread 11-12-2010   #10 (permalink)
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sounds like you have two problems, there is still air in your lines, and the piston in the master cylinder is not centered
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Unread 11-12-2010   #11 (permalink)
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I did bleed the master and when bleeding the brakes I start at the wheel farthest from the master cylinder, right rear, then left rear and so on. I've done that about 7 different times. I bled the master cylinder according to the instructions that came with it, twice. I'm probably going to pull it off and try bleeding it again.
I have a brand new ( not rebuilt) master cylinder. I had this same problem before I replaced the master cylinder and after replacing the M/C there was no difference at all.
The rear shoes are on correctly Ive checked that a bunch of times, I've also adjusted the shoes.
I guess there is just air in the system somewhere that I can't manage to bleed out I will keep trying
I appreciate all the help!
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Unread 11-12-2010   #12 (permalink)
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Well i rebled my master cylinder and bled the whole system more and it got a little better, then i forgot to tighten down one of the lines going to the master cylinder and hit the brake pedal... 3 hours of work ruined in a second. now its full of air again.
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Unread 11-15-2010   #13 (permalink)
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Re-bled the master cylinder and all four wheels and it feels a little bit better but its still weak, its kind of weird. Occasionally while stopping the brakes will work great at first then fade out.
also i noticed that when pumping the brakes with the reservoir cap off you can see fluid "squirt" up in the front reservoir (which ironically operates the rear brakes) and it only does this sometimes in the rear reservoir and it is not nearly as strong...
i hope this information helps someone point toward my problem
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