Spongey brakes cant find the problem

[Kultin]

Cant find the problem. Brake pedal is soft and spongey. It has been like that since i owned the car for 10 years now. Bought it with 4 wheel drum brakes and didnt run. Fast forward to when i got it running, had k member and front disc brakes from a 1972 charger. Rears were changed to right stuff disc brake. New master cylinder and proportioning valve were from the charger along with front brake lines. Pedal is soft and spongey. Can go to the firewall and doesnt have and end point. It will stop the car but will only lock up the front brakes on rock after fully pressing the pedal 3 times with each time gaining more braking power but still same pedal feel. Cass from doctor diff said to change the proportioning valve to distribution block and add a seperate variable proportioning valve. I purchased the recommended master cyclinder from Cass as well to help narrow problems down. I have bled and bled brakes. Even had some friends that are mechanics to bleed them as well to the same problem. The pedal rod with master work good in conjunction with each other. Do yall think I need check valves. I'm about to spend the best money and buy a complete brake kit, front and back with new lines because im tired of messing with this

 

[Kultin]

Also forgot to add, I bled the rear brakes per manufacturer. Take calipers off and have bleeder parallel to the ground while bleeding. I don't think I will ever buy Right Stuff brake conversion kit again. It's a pain to put the caliper back on and not cross thread the pin to bracket.

 

[RJRENTON]

Also forgot to add, I bled the rear brakes per manufacturer. Take calipers off and have bleeder parallel to the ground while bleeding. I don't think I will ever buy Right Stuff brake conversion kit again. It's a pain to put the caliper back on and not cross thread the pin to bracket.

Is your master cylinder bore size correct for the front calipers you are using? Also correct size master cylinder reservoir correct. What you show in your pixs doesn't look like a Mopar unit but one supplied with a conversation kit. Bore size directly affects the applied pressure to the calipers (and rear wheel cylinders) and how much squeeze on the calipers and therefore braking or clamping pressure on the rotor. Is the front rear proportioning valve operating correctly? Is the peddle push rod set to correct dimensions. Is the residual pressure valve in the master cylinder ok? Just a few points to ponder.....
BOB RENTON

 

[Mackman]

Don’t abandon what you have for another kit. You could end up with the same viscous circle of problems to solve. Resolve what you have.

 

[Kultin]

Is your master cylinder bore size correct for the front calipers you are using? Also correct size master cylinder reservoir correct. What you show in your pixs doesn't look like a Mopar unit but one supplied with a conversation kit. Bore size directly affects the applied pressure to the calipers (and rear wheel cylinders) and how much squeeze on the calipers and therefore braking or clamping pressure on the rotor. Is the front rear proportioning valve operating correctly? Is the peddle push rod set to correct dimensions. Is the residual pressure valve in the master cylinder ok? Just a few points to ponder.....
BOB RENTON

Bore for the master is 15/16. That is what Cass from doctor diff said for the best line pressure. The previous new master was one for a 72 charger that had a bore of 1.04in and pedal felt the same. Pedal pushrod has no extra play and I believe utilizes the full stroke, I can measure the stroke to make sure. It is just a distribution block and only a knob style for the rear. I have adjusted the knob and I believe it is working ok. I do not know if this master has a residual check valve.

 

[dvw]

I'd like to see a picture of how each caliper is mounted. Are the bleeders at the top? Have you watched the pads while someone else presses the pedal? The amount of pad movement should be minimal. If the pads are binding, caliper not square, or caliper spaced incorrectly it will have excess caliper piston travel. A 15/16" master with 4 large caliper pistons is pretty small. Takes a lot of pedal travel to displace enough fluid to fill them all. Master cylinder selection is touchy. 1/16" bore can make a difference.
Doug

 

[MoparLeo]

Common with 4 wheel disc conversions with mismatched parts. One thing that helps is to replace all the rubber lines with quality, braided steel lines. They won't expand under high pressures.
Easy, cheap solution. Put drum brakes back on the rear.
Don't need disc on the rear unless this is a road race car.
In that case you would replace everything in the brake system with matching, high performance components.

 

[Don Frelier]

If the pedal comes up after pressing it 3 times then I would strongly suspect air in the system.

 

[Kultin]

I'd like to see a picture of how each caliper is mounted. Are the bleeders at the top? Have you watched the pads while someone else presses the pedal? The amount of pad movement should be minimal. If the pads are binding, caliper not square, or caliper spaced incorrectly it will have excess caliper piston travel. A 15/16" master with 4 large caliper pistons is pretty small. Takes a lot of pedal travel to displace enough fluid to fill them all. Master cylinder selection is touchy. 1/16" bore can make a difference.
Doug

If you want, I can post pictures of front and rear calipers mounted. Yes the bleeders are at the top and the rears from Right Stuff say to bleed with the bleeder parallel to the ground. I will check each caliper for movement when I get a chance. So far from what I have researched, I may try a 1" bore master to add a little more volume and hopeful to stiffen the pedal some. I will try to Google one. Also maybe plumb the brakes direct to master with out distribution block to see if that may help

 

[Kultin]

Common with 4 wheel disc conversions with mismatched parts. One thing that helps is to replace all the rubber lines with quality, braided steel lines. They won't expand under high pressures.
Easy, cheap solution. Put drum brakes back on the rear.
Don't need disc on the rear unless this is a road race car.
In that case you would replace everything in the brake system with matching, high performance components.

Braided lines seem like the next step. I already had the 72 charger for parts. And was getting the rear-end rebuilt so I figured why not. But sadly the shop that did it, lost some parts for the rear drums and paperwork for all the new parts. Thank you for the insight

 

[Kultin]

I'd like to see a picture of how each caliper is mounted. Are the bleeders at the top? Have you watched the pads while someone else presses the pedal? The amount of pad movement should be minimal. If the pads are binding, caliper not square, or caliper spaced incorrectly it will have excess caliper piston travel. A 15/16" master with 4 large caliper pistons is pretty small. Takes a lot of pedal travel to displace enough fluid to fill them all. Master cylinder selection is touchy. 1/16" bore can make a difference.
Doug

The rear bleeder is behind the arm

 

[Kultin]

All calipers seem to be aligned. The rear brake hoses have a lot of movement. Front brake hoses do not move.

 

[dvw]

This style rear GM caliper does not self adjust caliper piston travel. Hit the E brake a bunch of times. This will ratchet the caliper piston outward closing up piston to rotor clearance.
Doug

 

[MoparLeo]

Sometime you can get air bubbles trapped in the calipers. And old fix is to tap the calipers with the bleeder open to force any bubbles to the top.