Replacing the Master Cylinder (with a Mancini Racing Kit)

[Grabinov911]

Because I looked all over the site and couldn't find the answer to this question, I post this:

The 1970 Roadrunner Manual Drum car has a master cylinder with 2 reservoirs. The reservoir in the front (toward the front of the car) serves the rear brakes. The reservoir toward the rear of the car serves the front brakes. The fitting size and thread pitch for the rear brake line (front of master cylinder) in 9/16" 20. The fitting size for the front brake line (toward the firewall) is 1/2" 20. Why are they different? I suppose to keep the assembly line folks from getting them backward. This is a serious pain, because 9/16" 20 is the rarest thread size known to man (based on my survey of every parts store within 100 miles of Los Angeles)! See pic below of factory Master Cylinder.



Getting this master cylinder out is a serious pain as well. It was installed before the dashboard went into the car! It is held to the firewall by 4 studs attached to the master cylinder which stick through the firewalll to the interior. The upper right nut (when sitting in driver's seat) is WAY up there, and hidden by the steering column and the dash structure. I used the 1/4"-drive socket combination below, with a 1/2" semi-deep socket, to get that nut off the interior firewall. It works for the other 3 nuts as well. There is a tape measure in the photo because the length of the socket itself (2") is relevant. Too long - won't fit. Too short - won't fit over the threaded part of the stud which sticks out beyond the nut. I used a snap-on flex head ratchet (also relevant) because the snap-on tools have a very fine wratcheting mechanism which lets you get lots of "clicks" in a small space (also relevant). It only took me about 3 hours to find this combination.



The Mancini Racing part is nicely machined aluminum, and it is threaded the same in both the front and rear ports. Both are 1/2" 20. This is a nice common size. It mounts to an adapter (black aluminum in pic below) which they sell which converts the four hole firewall to a two-stud mount. Strangely, while the part fits perfectly, they do not provide the four bolts which go through the firewall. The studs from the factory master cylinder (see pic above) cannot be used as they do not fit through the Mancini adapter holes - unless you want to machine the holes on your brand new adapter. You'll see that I put a spot of glue on the heads of the bolts I used to hold them in place as I tightened the nuts from the interior side. This worked well, but studs from the engine compartment side would have been the right technique - if the adapter came with them.



You'll see that it took an adapter from the 9/16" 20 ferrule on the factory brake line to the 1/2" 20. After extensive searching, the only thing I could find close was a 9/16" 18 to 1/2" 20. These were available at every store and all over the place online. Unfortunately this is NOT the size I needed. SO: I tapped the 9/16" 20 with the ferrule on the car!

Why?

Because a 9/16" 20 tap is the next rarest thing on the planet! Think that's bad? Try finding a 9/16" 20 BOLT to use as a tap (the fittings are only brass after all - easy to tap with a steel bolt)!

In the end I gently threaded the adapter fitting with brake fluid as a lubricant (hey it was dripping all over anyway right) - on, off, on, off, and eventually got a nice fit. As I said, at least brass is soft and easy to thread.

Oh, and one more thing!



I bought a Mancini adjustable length brake pushrod to go with my new master cylinder. In the picture above the black part is the Mancini adjustable pushrod end. You'll see that it looks nothing like the factory end. This MATTERS, because the pushrod "face" at the pedal (pictured) is the surface which contacts your brake light switch. The round tipped Mancini part does not work well against the round faced switch plunger. In addition, the adjustable part was nearly an inch too long (even after I cut off some of the threaded adjuster rod on it) AND the hole in the brake pedal end (pictured) is too small for the complex and important factory bolt/stud which holds the rod to the pedal. It looks the same size in this picture because I already needlessly machined this one to fit! (read on)

If you like, fight with it for two hours, cut the rod, machine out the hole in the end to fit the brake pedal stud and try to install it to find that it is STILL too long and wont work your brake light. or JUST USE THE FACTORY BRAKE PUSHROD! I wish someone had told me that.

Getting the pushrod out of the factory master cylinder is tough. Basically you just pull it out, but it is held in by a rubber bushing which does not give up easily. Just pull. replace the thrashed bushing with the one in the master cylinder kit. The push the factory rod int the Mancini master cylinder.

Hopefully someone searching the site will find this useful someday...

Greg

 

[jusglazin]

Thanks for the info. I'm installing a disc brake kit from Right Stuff now and your pain will come in handy.

 

[soundhd]

I too thank you for the update and the photos.... I plan on doing the same thing to the 70 RR I just bought........replacing the master cylinder & brakes with an aftermarket disc brake system

 

[67 coronet]

just for info mancini makes an adapter that has studs instead of bolt holes.did you check to see if you could replace the whole brake line down to the prop valve to change the end nut?

 

[Grabinov911]

From what I can tell, the Mancini adapter is described as having studs because it has pressed in studs (2) to hold the master cylinder. Their master cylinder and most aftermarket ones have only two mounting holes instead of four like the factory Chrysler master cylinder. So while the Mancini adapter does indeed have studs (for the master cylinder) it has bolt holes for the firewall mount. I called once I saw the one they sent me and they said the one I used was the only one they had. Could be wrong though I suppose...

I didn't think to replace the whole brake line down to the proportioning valve. That would have been a good idea. I'm considering starting a thread on brake bleeding because having done all of the above, I just CANNOT get the brakes to bleed. It takes 5 pumps to get anything out of the brakes - I assume because there is air in the system. I have tried twice on all four wheels, but I just can get it to work. The bleeder screws are SO short that I can get a wrench AND a bleeder hose on at the same time, and I just get air from around the bleeder nipple of the bleeder threads, even using a vacuum bleeder. I was wondering if all the loops in the brake lines near the proportioning valve make the car require some special bleeding technique. Brakes are NOT my area of expertise...

 

[67 coronet]

did you bench bleed the master before you put it on the car?if you dont get all the air out of the master before you install it can cause alot of trouble while trying to bleed the system since the pedal wont drive the plunger all the way to the end of the master and will leave an air bubble inside it.try leaving all the bleeder screws a bit loose for a while and see if you can get it to start a gravity bleed.

 

[Grabinov911]

I did bench-bleed the MC before I put it in, and thought that I did a pretty good job, but I certaily could have missed some air. I should add that the reason I replaced the MC in the first place was that after bleeding the brakes twice to no effect, I figured that the bad pedal feel/action was possibly due to a bad MC.

The last thing i tried Saturday was gravity bleeding. I let it run until i had good clear fluid from all four corners, about 3 ounces per brake. And it didn't make any difference at all.

Next, I may order a positive pressure bleeder (pump instead of vacuum) and try bleeding them UP from the brake rather than down from the MC. It takes 4 pumps almost to the floor to get the pedal even a few inches up with a solid feel, so we've got to be talking about a fair amount of air. I just CAN'T find it!

 

[Grabinov911]

Arghh!!!

Uhhh..... You mean to tell me that those little gear-shaped parts called "adjusters" actually have to be adjusted?

So my buddy stops by to help me bleed the brakes AGAIN after all the pedal-pumping problems I've been having, and after I bled (sp?) them 3 times by myself. We go to pull the first of the drums off and he says "you have no friction at all between the drum and the brake shoe. Are all the wheels like this?"

My response was something to the effect of "Of course they are. I don't want the shoes rubbing on the drum creating heat! Do I?"

You guys probably know this (I wish you'd said something but you may have simply avoided stating the obvious for fear of making me feel uneducated), but the brake shoes need to be adjusted via the star-wheels such that there is some (even significant) friction between the shoe and the drum. Once this tight, the master cylinder (which is a very small bore hydraulic cylinder which moves very little fluid) can cause enough movement in the piston within each brake to move far enough to apply the brakes. If the shoes are not tight enough to begin with (prior to the pedal being pushed), you will have to pump the pedal, not because of air in the line, but because the small master cylinder piston cannot move enough fluid in one press to move the shoes all that way!

I guess this is called "learning"...

 

[67 coronet]

glad to here you found the problem.never even thoght about the mechanical adjustment.like you said,i assumed you already did that.

 

[WV Roadrunner]

Very Helpful - Thank You !!!!

 

[Cojohnso1]

Yes it's an old thread. But good place to whine. I replaced my manual brake master cylinder. And that seemed to stop the slow introduction of air into the system. But my neck is killing me from a 4 hours job under the dash.

I rarely have complaints of Chrysler design on the cherished 68-70 B-bodies. But I'm VERY sure they could have done a better mounting design for manual brakes. With clutch pedal? It was a PAIN! with some bizarre wrench setups.

 

[themechanic]

Yes it's an old thread. But good place to whine. I replaced my manual brake master cylinder. And that seemed to stop the slow introduction of air into the system. But my neck is killing me from a 4 hours job under the dash.

I rarely have complaints of Chrysler design on the cherished 68-70 B-bodies. But I'm VERY sure they could have done a better mounting design for manual brakes. With clutch pedal? It was a PAIN! with some bizarre wrench setups.

Been there done that. Makes me appreciate being a tall midget.

 

[Cojohnso1]

Been there done that. Makes me appreciate being a tall midget.

I remember hearing someone say to take front seat out? I thought "Yeah right " But now I'm wondering?

 

[themechanic]

I remember hearing someone say to take front seat out? I thought "Yeah right " But now I'm wondering?

It really helps. But original seat studs may be a problem as they're exposed to the elements under the car.