Soft brakes after rear disc conversion

Bvh56 · Post by **Bvh56** » Sat Sep 21, 2013 12:16 pm

Hi all. I just got through converting my rear brakes from drum to disk. I used weld on brackets, chevy c20 calipers and dodge rotors. Everything went on good but I'm having a problem with a soft/spongey pedal and the rear tires lock up when I brake hard. Also the rear brakes seem to be dragging after braking. The truck is a 68 f 250 Cs that has front disc with power. So my question is where do I go from here? Do I need to change the proportioning valve? Or master cylinder? Here is some pics of what I have on the truck now. Also I did clamp the rear brake hose and and tried the brakes. The peddle was much firmer and both front brakes were engaged.

HIO Silver · Post by **HIO Silver** » Sat Sep 21, 2013 1:32 pm

The reason the rear is locking up is because the rear end is lighter than the front. You need to install a prop valve for the rear brakes... or a distribution valve which is tuned for a disc/disc combination.

I'm very surprised that very important detail was missed in advance of undertaking the rear disc conversion.

Bvh56 · Post by **Bvh56** » Sat Sep 21, 2013 2:48 pm

HIO Silver wrote:The reason the rear is locking up is because the rear end is lighter than the front. You need to install a prop valve for the rear brakes... or a distribution valve which is tuned for a disc/disc combination.

I'm very surprised that very important detail was missed in advance of undertaking the rear disc conversion.

Well everything I read there was no mention of changing the valve so I figured I would give it a try with what I had. Do you think changing the valve will help with the soft peddle?

crewcabfan · Post by **crewcabfan** » Mon Sep 23, 2013 4:32 pm

when you installed the calipers did you make sure the bleeder is at the highest point?
the first 4 wheel disc brake conversion i ever did (twenty some years ago ) i made the mistake of installing the caliper in their opposite sides and couldn't get rid of some air pocket inside the caliper until i swapped them back.
do you have any pics of the conversion? I personally don't think that the proportioning valve would cause such an issue but more like air in the system.

1972hiboy · Post by **1972hiboy** » Mon Sep 23, 2013 5:16 pm

Both of the suggestions offered are very valid. you need to do something with your proportioning valve to cater to your disc disc setup thats a fact. But the spongy pedal could be a few things. checking the calipers to make sure the bleeder is on top vs bottom is very important as well.

Bvh56 · Post by **Bvh56** » Tue Sep 24, 2013 7:30 am

Crewcabfan, maybe it's a bad rear end I got from you, haha just joking. But yes the bleeders are straight up in the 12 o'clock position. I did bleed them again and the peddle felt a little better. After reading and researching for hours I have decided to redo the system from the master cylinder down. I got a master for a 77 Lincoln disc brake car and a wildwood adjustable proportioning valve. I will update you all in a few days on the after install. Thanks Brad

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Soft brakes after rear disc conversion

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