Disc Brake conversion power master cylinder question

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Mike69cuda

Mike69cuda

Mopar Moron
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My 69 Barracuda has an SSBC power disc brake conversion. It still has drum brakes on the back. It is a corvette master cylinder and a GM brake booster with an adjustable proportioning valve on the rear brake line. This was installed by the previous owner.

The rear wheel cylinders were leaking, so I have been replacing the rear brakes. I have the booster and master cylinder off to get the steering box out.

It looks like it had a manual brake master cylinder on it with the deep pocket for the pushrod. The push rod was too short for the pocket, so it wasn’t doing anything. The booster ran was contacting the MC piston to operate the brakes.

I called SSBC and they recommended a corvette power brake master cylinder with a shallow piston that would work with the booster correctly. They seem to know their business, but my question is about the residual pressure valve.

Since the corvette MC is made for front and rear disc brakes, it doesn’t have a residual pressure valve. Do I need one? From what I have read, they were eliminated in all modern cars with rear drum brakes.
 
I think it’s supposed to come with a little adapter. I have the same thing but I’m thinking of using a different mc, this one is 9 lbs haven’t weighed the booster yet.

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Mine didn’t have the plug you have. I think that is a manual brake master cylinder.

I bought a power brake master cylinder that has a shallow bore, so I don’t need the plug now.

I am still having issues.

The pedal goes all the way to the floor with the master cylinder off. The booster appears to respond to the entire length of pedal travel.

However, with the MC installed and the brakes bled, the pedal only has a few inches of travel. The car stops, but you can’t lock up the brakes. The pedal feels like it hits a mechanical stop, like there is a brick under it. Very puzzled. Still investigating.

How far along are you?
 
I just pretty much finished under the car. I still have the interior to do, it’s bare at the moment. Windshield, dash, wiring,headliner, carpet,etc.

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Hi, very nice car.
I'd adjust the rear brakes, then set the emergency, re-bleed, I kinda suspect the master may be running outta travel moving out the rear shoes.
We recently went thru this, here's a similar thread that took the dude down a rabbit hole, but the rest a lotta good stuff there.
I didn't follow till the end, as the fella wasn't responding as expected.
Good luck, - I would expect more questions, lol
Don't go down the hole,
www.forabodiesonly.com

Power Disc Brakes not working on hard stop?

OK based on discussion and your success without power brakes, I've, disconnected the brake pushrod - Pedal to Booster and travel of pedal is much greater than when connected - so not pedal. So given everything else the limit has to be between power booster and MC. Life is short and I need to...
www.forabodiesonly.com www.forabodiesonly.com
 
Did you turn the adjuster nut in the power booster to meet up with the inside of the master cylinder. I just replaced my 4 bolt master to a 2 bolt. If you don’t get the exact measurement for the nut length you will get the problem you are having.
 
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It was this style. My old one was leaking out the back and the top. I replaced it at Muscle cars at the strip in the parking lots at the car show. Immediately felt improvement.
 
I'm having the same issue, sort of... i have 4 wheel drum, booster, and dual res m/c. New wheel cylinders, rubber lines, shoes, drums, and the pedal pumps pretty stiff but the brakes just don't stop the car. I put it on my lift, start it up and put it in gear and the rear spins away, apply brakes and it will slow the wheels but with full pressure on the pedal the wheels still move. Pulling my hair out over here! Any thoughts will be helpful !

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Do you have enough vacuum. Is your pedal pushing the MC all the way in. There is an adjustable nut in the booster and also adjustable at the pedal to booster.
 
loganscuda said:


FYI. just in case you didn’t know. I didn’t. You can use aluminum foil also or Clay
Click to expand...

Thanks! I have that tool, pushrod is set correctly. Even with hard pedal pressed all the way in I can still grab a hold of the front tires and spin them (not easily, but they do move)
 
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