Brakes have me stumped

I did a rear disc brake conversion on my challenger last summer. I have bled the brakes repeatedly with my pressure bleeder to the point that I have probably pushed through a gallon of fluid with no improvement. I still have a soft pedal. My brakes work but the pedal is spongy. I went out today and took each caliper off and bled them again with the bleeder pointing up so there could be no air trapped. Ran a quart through and the same spongy pedal. My master cylinder is new and was thoroughly bench bled before installation. I am completely stumped...Any ideas? This is the only issue I have with my car. If I can get this resolved y project will be completely finished...For now.

my guess is you probably need a 15/16 bore m/c.

Please elaborate...

do you have manual brakes?

WE have found that with dual M/C systems the following procedure works.

Bench bleed the master.
Install the master with both ports plugged.
Remove 1 plug and attach the corresponding line. Before trying to bleed the "SYSTEM" have a helper depress the pedal while Loosening said line. Close the line while fluid is still being pushed out of the master.
repeat 2 or 3 times.
Proceed to bleed that side of the system.

Remove the plug from the second port. Repeat above process.

My only explanation on why this seems to work is that the M/C is drawing air back into itself from the lines. DUNNO??? seems to work here
The only other process that seems to work is to bench bleed the master, Install it and pressure bleed the system

Hope this helps, brake problems Draw a perfect vacuum ( IE: they su-k) LOL

hope this helps

I have power brakes... I did bench bleed the MC and I have a pressure bleeder. That's what I have used to push all the fluid through with.

Cap the MC with a pair of brass inverted flare male plugs (fittings) from the Edelman cabinet at your parts house. If the MC is good & bled out it will be rock hard with virtualy no travel (try light and try hard pressure). then hookup ONE line & see if it still good. if so the air is in the other line. If not good then remove that line from the MC & cap that MC port back up & & hookup the other line & see if it is good or not & if not then both have air & start on that line (half) bleeding it out & as said dont let air come back in thru the bleeder screw threads on the return stroke (make sure your helper has em snug befor he hollers for you to let up on the pedal). You might agitate the fluid when doing the front discs by pumping the pedal rapidly several times then he opens a bleeder on the 4th downstroker (good verbal timing needed). then work on the other half. Put a block of wood etc under the pedal so you dont go past the normal travel range. If the MC is high and tight then the problem has to be at the corners & calipers act up way more than rear wheel cyls. holler how it progresses

power brakes give people the impression something is wrong.

pull the vacuum assist and see if they still feel spongy.

if the still do, then try robert's plan on figuring out where the issue is.

I had a similar problem when I installed the TSM rear discs on the hot rod. That install really gave me fits. I tried multiple master cylinders and a small tanker ship worth of fluid.

I took the lid off the master cylinder, pulled the rear calipers off, and opened the bleeders. While gravity bleeding I held each caliper in my hand to make sure the bleeder was absolutely the highest point of fluid contact. The way the calipers were cast it was a bit misleading.

Bleeding them like that solved the problem.

what brand of disc brake are you using

Could you have a problem with your proportioning valve? A semi-plugged or restrictive proportioning valve could produce similar symptoms.

Didn't know it, but I had absolutely zero rear brakes for around five years on a CJ Jeep I owned, due to a missing spacer/gasket in the proportioning valve. Wondered why my rear brakes never seemed to wear any.

During those years the Front brakes seemed to do the job, as 70% of braking is in the front, but was such a surprise when I discovered the fault.

It's taken me a bit to get back on this. So, I bled the dog crap out of the rears. I am certain they are fine. The front brakes are a bit different. If I put my pressure bleeder on and opened both front caliper bleeders and got nothing. I took the bleeder completely out and nothing! I put my vacuum bleeder on the front in conjunction with the pressure bleeder and a little bit came out. I got about a quart through each caliper but it took awhile. I still have spongy brakes. Just a refresher: I started out with disc up front and drum in the back. I put new calipers in the front and did a disc conversion in the back. Could it be my proportioning valve? The brakes were fine before I did the conversion and replaced the clappers in the front. Unfortunately, I did everything at the same time so I don't know where the problem originated. I can pretty much guarantee there is no air in the rear. I was going to block each line off but I didn't have anything they had the right threads to do it. At this point I am pretty sure its a problem with the front. I just don't know what the root cause is.

at this point me I would cap the front lines AT the calipers (might take some doing to get the right fittings) & see if the system is now high and tight. (if so) that'd tell me the air is in the calipers & some are a bear to tilt em just right to get the air to come out at 12 o'clock. EDIT I'd clamp the front hoses with (2) vice grips with protection of course for the serrated edges & you'd know in minutes the answer. Might not be the best for the hoses but I dont think it'd wreck em

I'm suspecting your directional valve may be off center thereby blocking the fluid to the calipers. The directional valve is often mis labeled as a proportioning valve. It's job is to block off 1/2 of the system when it detects a loss of pressure in that 1/2 of the system. It also triggers the brake warning light when the valve goes off center.

You can try having a helper put pressure on the pedal. Crack the port going to the front brakes, you should detect fluid being displaced under pressure. Tighten the fitting before allowing the pedal to come back up.

If that checks good move to one of the front output lines and repeat.

Continue through the system until you find the restriction.

Many times when I get problems like that I go around and block off the lines to the calipers as RapiRobert said. In a hurry you can use needle nose vise grips and block off both front at the same time. Then switch to the rears if no better and see if you get a good pedal like that. If the pedal gets solid by blocking the front or rear then block just one to see if its both or one side. Many times in conversions I see like Feets said that the bleeder on the caliper is not high enough and wont always get all the air out. But blocking the caliper lines will tell you if it has air stuck in a caliper. If I do the front and back and still no pedal you can do all 4 and see if it gets a pedal to know if its a problem in a front and rear caliper. I have found many problems like this including a Ford that had the calipers on the wrong sides and that caused the bleeders to be to low and would not bleed all the air out. Good luck , Ron