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#929320 - 04/24/12 06:29 AM Rear Disc Conversion Issues  
Joined: Mar 2012
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Rip1976 Offline
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Rip1976  Offline
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lake City, AR
Ok here is the whole setup

New "well refurbished" autozone master cylinder
Blazer front calipers
Metric back calipers

As of right now the front lines are still connected to the porportion valve but on the rear I connected it directly to the line coming from the master cylinder with an adapter since I plan on adding an adjustable proportion valve wednesday when it comes in.

Now for the problem!! The fronts bleed out just fine an seem to be working with no problems. The rears seem to be getting fluid to the calipers they will bleed out when the bleeders are opened But the peddle never goes all the way to the the floor when bleeding the backs. Actually it never seems to move at all.

I think I have the lines ran correct but I'm not 100%. I have the big line "front closest to the radiator" coming from the master cylinder going to the rear brakes an the small line "closest to the firewall" from the master cylinder going to the front brakes.

Anyone have any ideal whats going on or wrong!?!?

#929701 - 04/26/12 07:37 PM Re: Rear Disc Conversion Issues [Re: Rip1976]  
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Rip1976 Offline
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Rip1976  Offline
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lake City, AR
I returned the master cylinder an got another thinking it was faulty an even rerouted the rear lines back through the factory proportion valve an its still acting the same way. Anyone have any ideals or input???

#929705 - 04/26/12 07:44 PM Re: Rear Disc Conversion Issues [Re: Rip1976]  
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SickSpeedMonte Offline
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SickSpeedMonte  Offline
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MD
I was having a conversation with Adam the other day about this. PM Sixtoes1313. He told me the answer but I can't remember it. It's something to do with the proportioning valve if you put new rear wheel cylinders (or in your case calipers) on.

#929706 - 04/26/12 08:10 PM Re: Rear Disc Conversion Issues [Re: SickSpeedMonte]  
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Tony_SS Offline
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Tony_SS  Offline
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Washington, MO
I put rear discs on and am running the factory booster, master and prop valve... no issues.

#929800 - 04/27/12 12:37 PM Re: Rear Disc Conversion Issues [Re: Tony_SS]  
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SickSpeedMonte Offline
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SickSpeedMonte  Offline
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MD
To clarify, it was something to do with setting the prop valve a certain way while bleeding.

#929855 - 04/27/12 07:03 PM Re: Rear Disc Conversion Issues [Re: SickSpeedMonte]  
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Rip1976 Offline
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Rip1976  Offline
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lake City, AR
My adjustable came in yesterday but I haven't installed it yet. Gonna see if I can get my buddy over tonight with his air feed bleeder an see if it helps any. I'm still at a loss trying to figure this out

#929857 - 04/27/12 07:09 PM Re: Rear Disc Conversion Issues [Re: Rip1976]  
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Tony_SS Offline
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Tony_SS  Offline
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Washington, MO
I remember my buddy helped with my rear discs... said something about a ball getting stuck when bleeding the rears and if that happened it was bad.. but it didnt happen to us. I can ask him more..

#929868 - 04/27/12 08:33 PM Re: Rear Disc Conversion Issues [Re: Tony_SS]  
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richan Offline
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little rock,ar
It might depend on what master cylinder you got because with the stock MC IIRC there is a check valve to keep fluid in the rear brake lines so the pedal doesn't drop when you first hit the brakes. The disc need a lot of fluid compared to the stock drums.


87 MCSS Aerocoupe, 2 84 Elco V6 & V8, 87 Elco V8
#930850 - 05/05/12 12:16 AM Re: Rear Disc Conversion Issues [Re: SickSpeedMonte]  
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MADMIKE Offline
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MADMIKE  Offline
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C.V. CA
Take a gander at the second combo valve. That is what is found on 78-88 A/Gs and other GM RWD cars of the era.


Originally Posted By: SickSpeedMonte
To clarify, it was something to do with setting the prop valve a certain way while bleeding.

On the combination valves that are used in the 78-88 A/Gs there is a 'metering','delay', or 'anti-dive' pressure valve on the front circuit. All this does is prevent pressure from building in the front calipers before the rear drums begin to actuate.

On the front of the combo valve there is a shaft that you can pull forward using a clip that holds it open during bleeding. On a system that has brake fluid in it, this is not really needed to be done as brake fluid is not compressible and will push the valve open. However, if the front system has been drained or re-plumbed and there is a large mass of air in the system, the valve will need to be held open to aid in bleeding.

Originally Posted By: Tony_SS
I remember my buddy helped with my rear discs... said something about a ball getting stuck when bleeding the rears and if that happened it was bad.. but it didnt happen to us. I can ask him more..

He was probably referencing the PDV(pressure differential valve), which activates the brake light switch if there is a difference of pressure between the primary and secondary circuits. If a failure in one of the circuits caused a large leak and loss of pressure, the side that has not failed would still build pressure and shunt the PDV piston over activating the brake light.

I've never had this valve shunt over on me while bleeding brakes.

Originally Posted By: richan
It might depend on what master cylinder you got because with the stock MC IIRC there is a check valve to keep fluid in the rear brake lines so the pedal doesn't drop when you first hit the brakes. The disc need a lot of fluid compared to the stock drums.


Back in days of old, when men were bold...
With the MC below the floor board a residual valve was needed to prevent the fluid from draining back into the MC. With firewall mount MCs this wasn't needed anymore.
In the A/Gs this is not applicable.

Although it does matter what MC you use, you want one that is capable of actuating the calipers. Luckily the later G bodies used 24mm(that's between 15/16" & 31/32" in SAE) bore that can displace enough volume to correctly sized rear calipers ~38mm-45mm with 60mm front calipers.

However if the MC was changed to a different model and the pushrod length was not checked the it could be not fully releasing keeping the front brakes locked up.

However, from the OPs post it may simply be that the system was bled from front to rear which is incorrect.
Also verify that the bleed screws are pointing up or are at the highest point of the caliper. Left and right calipers can interchange, but you will never get the system to bleed correctly.


-Michael
Where the hell is JMD?!
#932865 - 05/21/12 02:28 AM Re: Rear Disc Conversion Issues [Re: MADMIKE]  
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Rip1976 Offline
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Rip1976  Offline
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lake City, AR
Well I think I finally figured it out. I had to undo the brake line on the right rear from the caliper an bleed just the line. After I got it to bleed out I was able to bleed out the caliper an got it work "I think anyway" guess I will find out when we finally get the car driveable lol.

#937975 - 07/02/12 11:59 PM Re: Rear Disc Conversion Issues [Re: Rip1976]  
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CYACOP Offline
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CYACOP  Offline
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Dallas, TX
Is the bleeder on the top or bottom of the calipers?


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