Rear disc brake conversion issues

[Erik72]

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,

 

[midlife]

Your master cylinder needs to change to a disk/disk configuration; the disk/drum configuration won't work with rear disks.

 

[Ed Raver]

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

 

[c9zx]

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

 

[tony-muscle]

+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-connecting-an-adjustable-proportioning-valve-to-stock-distribution-block

 

[Mister 4x4]

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.

 

[tony-muscle]

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.

 

[Erik72]

Just an update. The disc to disc master cylinder and combination valve combo from CJ pony parts cured the issue. Thanks to all for recommendations.

 

[c9zx]

Just an update. The disc to disc master cylinder and combination valve combo from CJ pony parts cured the issue. Thanks to all for recommendations.

I'm glad you got it sorted out. Chuck

 

[1973machi]

Just an update. The disc to disc master cylinder and combination valve combo from CJ pony parts cured the issue. Thanks to all for recommendations.

Did you hook up the brake light to the new prop. valve?  The factory one is two wire and the chevy valve CJ uses is a single wire.

 

[Erik72]

Just an update. The disc to disc master cylinder and combination valve combo from CJ pony parts cured the issue. Thanks to all for recommendations.

Did you hook up the brake light to the new prop. valve?  The factory one is two wire and the chevy valve CJ uses is a single wire.

I did yes works great

 

[1973machi]

Does it matter which wire I use from the factory plug?

 

[72Grande]

I bought the kit from summit...

I'm having the same problem. bought the kit from summit. bleed the brakes so many times and nothing. Thought maybe master cylinder went out so bought power booster/MC kit from LEED brakes. So now I have all brakes converted to disc. Worked fine before installing kit SUM-BK1626.

Emergency brake cable doesn't reach plus other problems.

Watching this thread.

 

[Don C]

The problem was solved, read all of the posts, especially about proportioning valves and about calipers installed on the wrong sides.