1970 Disc brakes master/booster rebuild - what am I doing wrong ?

Chris Many

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Hi Guys,

So I am rebuilding a master cylinder for my 1970 FM3 Satellite 318 Conv. It is an original power disc brake car. The master and booster were an unusable block of rust, so I located a matching master/booster assembly. same part numbers, built almost on the same day. 315th day of 1969.

The booster I had restored by Power Brake Booster in WA.

The master only needed a light bore and new seals, so I bought the correct NOS rebuild kit. I even confirmed all the part numbers with the parts manual. so I got the right rebuild kit and the right master.

Ok, so my problem is, is that the rear plunger sticks out the rear about 0.150", but it doesn't on the original, nor did it stick out on the rebuilt master before I took it a part. refer to pictures.

When I push the plunger in so that it is flush I can see that the opening is the same in the reservoir as the original (refer to picture).

I know that I am supposed to adjust the pushrod on the booster to have about 0.020" clearance with the rear piston in the master. Based on the original booster and the new booster, it looks like I really have a problem with my rebuilt master.

So, what is going on ? Do I just need to tighten up the spring on the rear piston or am I supposed to hold the piston flush while I use the booster pushrod alignment gage? The service manual is no help at all.

master compareB.jpg master compareC.jpg master kit.jpg master compareA.jpg New Boosterjpg.jpg
 

pnora

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There should be a stop pin that screws in from the bottom to keep the piston in. If that is in you may have the wrong kit for that master. Did you compare all the piston steps and length to the old?
 

pnora

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My 69 says this.

Push piston assemblies
into bore. Carefully work lips of cups into bore, then
seat piston assemblies. (Fig. 6).
(3) Holding pistons in seated position, install piston retaining screw and gasket.
 

Fran Blacker

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Don't know if this what your MC looks like? Set screw on bottom, Came from 70 SM.
set sc.JPG
 

Chris Many

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The pistons match exactly and I did install the stop screw correctly. I just acts as if the spring needs to be tightened down further, but the screw is already bottomed out.
 

pnora

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The pistons match exactly and I did install the stop screw correctly. I just acts as if the spring needs to be tightened down further, but the screw is already bottomed out.
Forget about any spring. You have something wrong with either the piston or the screw.
 

pnora

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The pistons match exactly and I did install the stop screw correctly. I just acts as if the spring needs to be tightened down further, but the screw is already bottomed out.
The step in the piston has to correlate to the stop screw. You need to measure old and new pistons. You also need to measure the screw length and copper gasket thickness. You could try putting the screw in without the gasket. Remember to push the piston all the way in and do not over tighten the screw trying it this way. The springs have nothing to do with the piston coming to far out.
 

Chris Many

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It's not the stop screw or the piston. everything is identical to the originals based on a side by side comparison and I installed them per the bendix instructions that came with the kit.

After a little bit more research:

Looks like my rebound spring isn't set right / compressed enough. It is shown in this video at the 12:04 mark.

MOPAR MASTER CYLINDER REBUILDING - YouTube

If the spring isn't set right the piston won't align with the port. I just need to turn the alignment screw down until the piston is aligned with the port properly, which I believe is shown in my second picture.

I've read on some older posts that some people adjust the booster plunger so that the piston aligns with the port. Know I think I get what they were talking about. They are compensating for the spring not being set right. But is that a good idea? That puts some back pressure on the booster, where there is supposed to be 0.020" - 0.060" clearance.
 

pnora

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It's not the stop screw or the piston. everything is identical to the originals based on a side by side comparison and I installed them per the bendix instructions that came with the kit.

After a little bit more research:

Looks like my rebound spring isn't set right / compressed enough. It is shown in this video at the 12:04 mark.

MOPAR MASTER CYLINDER REBUILDING - YouTube

If the spring isn't set right the piston won't align with the port. I just need to turn the alignment screw down until the piston is aligned with the port properly, which I believe is shown in my second picture.

I've read on some older posts that some people adjust the booster plunger so that the piston aligns with the port. Know I think I get what they were talking about. They are compensating for the spring not being set right. But is that a good idea? That puts some back pressure on the booster, where there is supposed to be 0.020" - 0.060" clearance.
That master in the video is not the same as yours. It does not use a stop pin into the bore. It uses a lock tab and screw in the butt end. The start of the video shows that threaded hole.
 

pnora

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One other thought as there are so many variables on master cylinders is that maybe the end of the piston does protrude slightly. If you measured the old vs new not just a visual. and it all goes together correctly that's maybe the way is is made. Your old one you are comparing to may be different or the piston is stuck and not coming all the way out. Because of these variables that is why they have the adjustable push rod on the booster.
 

Fran Blacker

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If you put MC together with piston flush with back. Put a pin in screw hole does the piston stay in place?
 

Chris Many

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So I concluded that the piston is supposed to stick out the back like my rebuilt master.
In the pictures you can see my original piston assembly and new one from the kit are identical. they even both have 749 stamped on them.
I measured the length of the original piston and spring from the location where the screw makes contact to the spring cup = 3-7/8"
I put a punch in the screw hole and measured the depth of the bore to the punch on the original master = 3-5/8"
adding 1/8" for the width of the punch, I get 3-3/4"
which means the piston should stick out the back about 1/8", which is does.

I adjusted the new booster's plunger to match the original. When I put the rebuilt master on the rebuilt booster I can see that the plunger is in the same position as the original one in the vent hole (refer to 2nd picture from my original post at the top). So now I believe I ma matching up to the original factory setup.

So this business about setting the booster plunger with an 0.020" clearance doesn't seem to apply to this setup.

original_master.jpg pistonA.jpg pistonB.jpg boosterA.jpg boosterB.jpg
 

pnora

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I figured that that's where the piston would fit in the master. As far as the booster rod adjustment goes I never went by any predetermined settings. I always measured what I had and set it accordingly. All you need is just a little clearance to allow the masters piston to fully return.
 
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