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Rear disc brake conversion issues
#1
Hey all,
Need some help. I recently did a disc conversion on the rear of my 72. She came factory with disc in the front and drums in the rear. Master and booster are original but were working fine with the drums on. I bought the kit from summit. The install was super easy. But now I need to pump up the brakes to get it to stop. I have bleed her so many times now I lost count. Definitely no air in the system. I blocked off the rear line to verify that the issue is in the rear and it is. As soon as I uncrimp the line peddle falls again. Any suggestions on what the issue could be?
Thanks for any help,

[color=#1E90FF][font=Courier]
Erik Passaro,

72--- 351c, Hooker Headers, 770 Holley carb, Steet dominator intake, comp cam, scorpian roller rockers, full MSD ignition, 9 inch 4-11 gears, 150 shot nitro.
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#2
Your master cylinder needs to change to a disk/disk configuration; the disk/drum configuration won't work with rear disks.

Let me check your shorts!
http://midlifeharness.com

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#3
I ran into the same issue when I changed over to rear disc brakes. I changed it all over to a hydroboost system and now works fine. Ed Raver
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#4
I'm not sure the items I suggest checking are directly related to the symptoms you described but worth considering. Make sure the rear bleeders are at the highest spot in the caliper. If not, L and R are reversed and it will never bleed all the air out. The pressures provided by the combination valve for rear drums is about half of what the discs need. There are "universal" combination valves with no pressure differential front to rear. This would be a plumbing nightmare as well as requiring an adjustable pressure valve in the rear side to set rear line pressure so the rear doesn't lock up before the front (very unnerving). Some people modify the stock combination valve so that it provides the same pressure front to rear. This also would require a separate adjustable pressure valve in the rear circuit. This may be in left field but, the drum side of the master cylinder, inside the outlet port, there should be a 10lb. residual pressure valve behind the brass sealing flare (for drum brakes). The brass flare can be removed with a sheet metal screw and pliers to gain access to the residual valve (it is a brass plunger with a rubber sealing edge). Since the brakes worked fine before the conversion it is probably safe to assume the booster pushrod length is correct. I hope you get it sorted out. Let us know what you find. Chuck
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#5
+1
After installing the proper master I suggest that you remove the proportioning plunger from the stock combination valve and add an adjustable proportioning valve. In this way you can adjust the rear brake bias.
This is what I did:
https://www.7173mustangs.com/thread-conn...tion-block

        [Image: 20160929_171923_edit2_small.jpg]

1971 M-Code Mach 1 w/Ram Air, 408 stroker, 285/291 0.558" roller cam, Blue Thunder intake, TKO600, Hooker headers with electric cut-offs, FiTech EFI w/ RobBMC PowerSurge pump
4-wheel disc brakes
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#6
(03-13-2019, 04:55 PM)c9zx Wrote: I'm not sure the items I suggest checking are directly related to the symptoms you described but worth considering. Make sure the rear bleeders are at the highest spot in the caliper. If not, L and R are reversed and it will never bleed all the air out. The pressures provided by the combination valve for rear drums is about half of what the discs need. There are "universal" combination valves with no pressure differential front to rear. This would be a plumbing nightmare as well as requiring an adjustable pressure valve in the rear side to set rear line pressure so the rear doesn't lock up before the front (very unnerving). Some people modify the stock combination valve so that it provides the same pressure front to rear. This also would require a separate adjustable pressure valve in the rear circuit. This may be in left field but, the drum side of the master cylinder, inside the outlet port, there should be a 10lb. residual pressure valve behind the brass sealing flare (for drum brakes). The brass flare can be removed with a sheet metal screw and pliers to gain access to the residual valve (it is a brass plunger with a rubber sealing edge). Since the brakes worked fine before the conversion it is probably safe to assume the booster pushrod length is correct. I hope you get it sorted out. Let us know what you find. Chuck

Bingo!  Chuck does it again.   thumb

I had the exact same issue when I first installed the new calipers on my fronts (stock/OEM style power disc brakes), and I'd gotten the calipers on the wrong sides with the bleeders lower than the lines, creating air pockets in the system.  Once I swapped them over to the correct sides, all was good.

Eric

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#7
(03-14-2019, 11:43 AM)Mister 4x4 Wrote:
(03-13-2019, 04:55 PM)c9zx Wrote: I'm not sure the items I suggest checking are directly related to the symptoms you described but worth considering. Make sure the rear bleeders are at the highest spot in the caliper. If not, L and R are reversed and it will never bleed all the air out. The pressures provided by the combination valve for rear drums is about half of what the discs need. There are "universal" combination valves with no pressure differential front to rear. This would be a plumbing nightmare as well as requiring an adjustable pressure valve in the rear side to set rear line pressure so the rear doesn't lock up before the front (very unnerving). Some people modify the stock combination valve so that it provides the same pressure front to rear. This also would require a separate adjustable pressure valve in the rear circuit. This may be in left field but, the drum side of the master cylinder, inside the outlet port, there should be a 10lb. residual pressure valve behind the brass sealing flare (for drum brakes). The brass flare can be removed with a sheet metal screw and pliers to gain access to the residual valve (it is a brass plunger with a rubber sealing edge). Since the brakes worked fine before the conversion it is probably safe to assume the booster pushrod length is correct. I hope you get it sorted out. Let us know what you find. Chuck

Bingo!  Chuck does it again.   thumb

I had the exact same issue when I first installed the new calipers on my fronts (stock/OEM style power disc brakes), and I'd gotten the calipers on the wrong sides with the bleeders lower than the lines, creating air pockets in the system.  Once I swapped them over to the correct sides, all was good.

To add to the bleeder orientation issues, some conversion kits use the populer Cadillac ElDorado calipers like mine. These have the bleeders facing up but they are not located all the way at the top of the caliper. That said if you don't turn the calipers during bleeding some air will get trapped at the top of the caliper. Hard to explain, but it is well documented for these caliper. You need to turn the caliper slightly so the bleeder is a the very top to allow all the air to escape. It is a sloppy design IMO.

        [Image: 20160929_171923_edit2_small.jpg]

1971 M-Code Mach 1 w/Ram Air, 408 stroker, 285/291 0.558" roller cam, Blue Thunder intake, TKO600, Hooker headers with electric cut-offs, FiTech EFI w/ RobBMC PowerSurge pump
4-wheel disc brakes
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