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Old 11-04-2012, 06:15 AM
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Originally Posted by davidmij

I was reviewing my posts this morning and realized I never posted what I found on my brake issue.

The company sent me a 9 inch power brake booster to replace the 8 inch - it didn't help,

...but now I was getting brake fluid bleeding out of the MC after driving the car and applying the pedal.
The 9" booster will give higher line pressures.

I googled around and found that a couple of things could cause this. One was a frozen wheel cylinder or caliper. If one of your wheel cylinders are frozen, it can cause extra brake line pressure which can cause your brake fluid to leak around your master cylinder cap.

I inspected each brake and found one of the rear drum cylinders wasn't working. This caused brake proportioning valve to cause excess back pressure and favor the front brakes.

I had previously checked the fluid pressure at each wheel but that doesn't show you if the wheel cylinder is working or not. After replacing it the MC no longer leaked. The brakes are much better now, but still not as good as I would have thought

Just thought I would update this in case anyone was using it.

regards, Dave J
Fascinating theory. You might check the MC mounting to see if the excessive pressure also caused bleed-by at the MC piston possibly causing mild hydro-static lock-up.

It may also be that the MC piston stroke is too long, possibly causing damage.

**** good diagnostics...
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Old 11-04-2012, 06:27 AM
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Originally Posted by ncbird View Post

David, take a look at the MBM typical configuration page they
Recommend a residual valve between the proportioning valve and the rear drums.

This might be a help in your/our situations. Since most of our upgrades use MBM parts I would think their advice should be considered.
I do not know what kit was used on this conversion, but if the MC was a GM large dual reservoir, the MC was designed originally for a DISC/DISC application. There is no residual valve as one is not used in a DISC/DISC application. The kit vendors use this MC as it can be included in either a DISC/DRUM or DISC/DISC changeover.

Any drum brake system has to have a 10# residual valve (GOOGLE for brake theory as I am tiring of harping on this).

The only kit vendor I have seen present this factoid is MPB. BTW- They also offer the correct FORD style MC (optional) in their kits.
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Old 11-04-2012, 10:00 AM
davidmij davidmij is offline
Join Date: Jan 17 2011
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davidmij is on a distinguished road

Hm, It's been a while and I didn't bookmark the page I read it on. What I typically do is save the notes to a file, here is what I had saved,
"Sounds like you may have a frozen wheel cylinder or caliper. Do you have drums all around or disc fronts? If one of your wheel cylinders are frozen, it can cause extra brake line pressure which can cause your brake fluid to leak around your master cylinder cap. I would inspect each brake and make sure that you do not have a problem with one of them. Also, a defective brake proportioning valve may cause excess back pressure too."

Other suggestions I found said that a refurbished MC can have an irregular top rim, (where the caps rubber seal lies). Most likely because the old one was rusted and pitted, and was not resurfaced properly for a smooth surface. Some guys on the link said to take it off and flat file the surface. Here's one of many that talk about this,

All I know is that I could bleed the brakes just fine, and a mechanic had told me I had good pressure at all wheels - I didn't ask where he checked that though. When I found one of my rear drum wheel cylinders not moving I replaced the wheel cylinder and it no longer leaked out the cap seal. The MC leak was from the smaller (rear brake) reservoir. It is a disc/drum setup.
The kit I bought was from "" . I believe it was the secnd one down on this page,

MBM couldn't get me one for a few weeks so I went with these guys - probably a bad idea, but they were helpful for the most part.

I had set the piston stroke several times, and when the guys who make the kit sent me a replacement one with the 9 inch booster I had them set the stroke - they said DON'T adjust it so I didn't.

Not sure what else I can tell you, all I know is that replacing the rear wheel cylinder fixed it.

Dave J
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Old 11-04-2012, 04:06 PM
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Red face

I don't think I made myself clear Dave...

What I was referring to was your saying you do not have enough brake effort. The mention of a residual valve was brought up and I just reaffirmed the need for that valve for proper drum brake response and pedal height.

Sorry. You know how bad war-time help can be...
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Old 11-05-2012, 10:03 AM
davidmij davidmij is offline
Join Date: Jan 17 2011
Posts: 660
davidmij is on a distinguished road

Well, after getting the replacement rearend in and back together, including bleeding the rear brakes, I now find that the MC IS leaking again - crap, this will never end!
I also read that over filling the reservoir can cause this, I also noticed that my rubber seal is not very uniform. I'll try to find out if I can replace just the rubber seal for starters - not sure if they sell them alone.

Gary, I don't remember anything about a "residual valve" from any posts. I may have thought that it was another word for "proportioning valve". But I had gotten to the point where I was chasing my own tail because I was totally lost. I'll try to look at the post and info at MBM tonight so I can see what you are talking about.

The only thing I haven't replaced is the proportioning valve. I'll post pix tonight of my prop valve, MC and booster, and lines.

Thx to all as always!
BTW, The new 3.89 rearend is fun! It's amazing what a difference it makes from a hole shot. I went out to a remote road and ran through the gears - speed shifting to second gear the tires spun and I got a lot of axle hop (chatter). Probably a good time to replace the 30+ year old shocks, and either replace or retention the springs.

Dave J
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