# Front disc brake conversion



## gtoearl (Dec 21, 2009)

Hopefully you guys can help me with an issue. Just replace the upper and lower control arms with new front springs. Also was able to score a disc brake conversion from a 66 Chevelle. Everything bolted on correctly. The items I purchased Included the master cylinder but without the proportioning valve and booster. I still have the rear drum brakes. Which I refreshed with new shoes springs and drums. Got a proportioning valve from C PP . The booster I purchased from Summit. Hopefully this is enough background for you to answer my question. 
I can’t seem to get the adjustment on the rear brakes to the point where they will not lock up . Seems to not lock up if I’m fairly easy on the brake. If I put the slightest increased pressure like a hard stop the rear brakes will lock up. 
A friend of mine from a local restoration shop told me they never put a combination proportioning valve on their brake conversions. They simply run the front reservoir to the front brakes and put an adjustable proportioning valve to the rear brakes. I’m not questioning their procedure but my question is why do all these kits out there have a combination proportioning valve for the brake conversion. I’m wondering if I have something wrong with the mechanical issue on the rear brakes, adjusted too tight, emergency brake adjustment or some other mechanical issue. I do not have any gauges to check the pressure to the front and rear. I’m wondering if I have a defective combination proportioning valve where it is not centered. I’m sure there’s many reasons why my brakes are locking up on the rear. But I would like to get some input from all you guys. Thanks for any help.


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## cij911 (Oct 25, 2017)

The master cylinder needs to have the different valve for drums, otherwise it will send too much pressure and lock the rears (what you are experiencing).... You can get the correct valve at CPP if Summit won't fix for you...


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## gtoearl (Dec 21, 2009)

Appreciate very much your response. I did not buy the master cylinder from a retailer. This was the master cylinder that came with the deal I got from the guy that owned the 66 Chevelle. I understand after calling the restoration shop where I got the stuff that the master cylinder was for a disc/Drum system.


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## cij911 (Oct 25, 2017)

gtoearl said:


> Appreciate very much your response. I did not buy the master cylinder from a retailer. This was the master cylinder that came with the deal I got from the guy that owned the 66 Chevelle. I understand after calling the restoration shop where I got the stuff that the master cylinder was for a disc/Drum system.


Hmmmm....assuming the plumbing is correct, then I would check pressure at the rear.


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## PontiacJim (Dec 29, 2012)

gtoearl said:


> Appreciate very much your response. I did not buy the master cylinder from a retailer. This was the master cylinder that came with the deal I got from the guy that owned the 66 Chevelle. I understand after calling the restoration shop where I got the stuff that the master cylinder was for a disc/Drum system.


Purchasing a "deal" from another does not mean you got a "deal" let alone the correct & matching parts. 

Are the items brand new and unused? If they were used, maybe the guy with the Chevelle was also having problems and decided to get rid of the system and go with something different. Seeing the '66 Chevelle did not have disc brakes and the dual master cylinder did not appear until 1967, what exactly do you have - factory GM parts or aftermarket add-on?

We have had posters on the forum buy a new set-up that was for their car only to have problems and not seem to get it to work correctly. It is very important to match all parts. Maybe something incompatible that doesn't match? :thumbsup:


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## iharting (Oct 20, 2018)

Can you post pictures of the Master Cylinder and Proportioning Valve? One of them should be the wrong part for that set, because it seems too high fluid pressure is going to the rear brakes.


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## gtoearl (Dec 21, 2009)

Hopefully this helps. The proportioning valve is from CPP. The master cylinder was from the 66 Chevelle set up.


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## gtoearl (Dec 21, 2009)

This is how I received it from the purchase I made. You can see that there’s a small I think adjustable proportioning valve plumbed in. I’ve been told this is not a proportioning valve.


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## cij911 (Oct 25, 2017)

gtoearl said:


> Hopefully this helps. The proportioning valve is from CPP. The master cylinder was from the 66 Chevelle set up.


That is what you want and you HAVE TO MAKE SURE IT IS FOR REAR DRUM SETUP. You many have received the wrong Part #.


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## iharting (Oct 20, 2018)

Ok. If proportioning valve was not centered, you would have a red brake light at your instrument panel ,so I would do this:

- Check if the rear drum brake shoes are in the right position (shorter shoes towards the front)
- Check if parking brake adjustment is not too tight (First adjust shoes with parking brake loose) 
- If problem is still there, check booster rod is not preloading the master cylinder piston
- Finally, try how the system works without the rear brakes residual pressure valve that is seen under the M/C

My '71 Chevelle had disc/drum brakes with proportioning valve and no residual pressure valve from factory and it worked perfectly. Then I converted it to disc/disc brakes with a new proportionig valve and no problems.

Work with patience.

Hope this info helps too:

https://www.brakebleeder.com/rear-brake-drag-lockup-drum-brakes/


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## BLK69JUDGE (Jun 10, 2010)

heres what my chicken sized brain sees ....

you need the stay off valve you got with the 66 stuff in the picture

the ccp thingee you have mounted now is probably just a distribution block

you need to plumb in the stay off valve ...


Scott


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## John Schutt (Aug 27, 2018)

I'm having the same issue on my '68 GTO. Had manual drums, bought a disc brake conversion kit from Right Stuff, rears consistently lock up on a hard stop.
Right Stuff calls it distribution block that was included. Valve is NOT tripped, put 12V test light clamp on Batt Positive, test light probe on distribution block sensor, no light up, it did light up when grounded to block.
any ideas?
Wrong Distribution block?
Thanks


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## gtoearl (Dec 21, 2009)

Well cij97. You are exactly correct in your advice. I took a close and hard look at all the parts and the plumbing of the master cylinder. Seems for some reason I had the reservoirs switched. I feel so stupid. Problem solved. Thanks for all your help out there.


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## gtoearl (Dec 21, 2009)

Meant to address my comment to cij911....


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