Fj55 Rear Disk Conversion (1 Viewer)

[WhiteMooose8]

Hi all,
I have a 1976 Fj55 that I am converting to rear disk brakes. I am using rotors from trail gear and GM Camaro calipers. I am running into issues with a spongy pedal that goes all the way to the firewall. I have the stock MC installed with new seals, and also verified that it was working properly pre-conversion. I have bled the system many times with the standard 2-person method and with a vacuum bleeder. The flow out of the bleed points is clear of bubbles every time. No matter what I do the vehicle stops slowly with a super soft pedal feel that goes all the way to the floor. The booster push rod seems to be set correctly as I found by capping off the line to the rear brakes and depressing the pedal, the pedal immediately stops moving without much free play.
Has anyone had experience with this, my next thought is to try a MC with a larger piston size to increase the volume to the rear.

 

[Pighead]

Did you position the bleeders so that they are uppermost?

 

[B y r o n]

I think it has been pointed out a few times that disc brakes require more fluid volume than drum brakes. It might be time for a 4 wheel disc master cylinder.

 

[Pighead]

It might be time for a 4 wheel disc master cylinder.

I have to disagree.
I went from the drums to adding front Toyota discs to adding Poser's rear discs all with the original '74 brake master cylinder. I don't think a new master will fix this.
I think he has air in the system.
If he has a good pedal with the rear blocked off, then the air is in the rear. Hence my question about caliper orientation.

 

[ceylonfj40nut]

I have to disagree.
I went from the drums to adding front Toyota discs to adding Poser's rear discs all with the original '74 brake master cylinder. I don't think a new master will fix this.
I think he has air in the system.
If he has a good pedal with the rear blocked off, then the air is in the rear. Hence my question about caliper orientation.

^^^This! Had the same issues. Bleeder was on bottom on one of the calipers. Check bleeders. Took it out, flipped it and placed a piece of metal into caliper to hold the pads while bleeding. Tons of air came out. Then installed it. Solid brakes.

 

[WhiteMooose8]

^^^This! Had the same issues. Bleeder was on top on one of the calipers. Check bleeders. Took it out, flipped it and placed a piece of metal into caliper to hold the pads while bleeding. Tons of air came out. Then installed it. Solid brakes.

Ok, just to clarify, I have both orientated so the bleeder is pointed directly up with the banjo bolt at the bottom. Is this what you found to be the best for a good bleed?

 

[ceylonfj40nut]

Ok, just to clarify, I have both orientated so the bleeder is pointed directly up with the banjo bolt at the bottom. Is this what you found to be the best for a good bleed?

Yes!

 

[WhiteMooose8]

Update: I flushed the system again and this time once it was totally clear I cracked the line in the rear before the rear axle to put a small bubble in the line hoping that if there was any other air in the system the bubble would find it and not come out the bleeder. I repeated this with both sides in the rear and saw the small bubble come out each time. At this point, I am reasonably sure all the air is out and I still have a soft pedal to the floor and can't get it to lock up. is it a possibility that poor caliper alignment could cause something like this? The wear marks on the new rotors look nice and even but anything could happen.

 

[Pighead]

I was hoping that you had put your calipers on backward and it would be an easy fix.
But a spongy pedal says air in the system to me. Did your master ever go dry? Did you bleed it?
What size master does a Camaro use?

 

[WhiteMooose8]

The master has been bench bled out of the car and then again once installed, This really feels like air to me but I can't think of anything else to try to bleed it out especially after capping the line at the axle and having a rock hard pedal. The GM masters seem to be between 1 and 1 1/8 inch bore depending on if the car/truck had a booster or not. not sure about the exact stroke length tho. It's strange because I've seen multiple swaps with the original master and reports of great stopping power.

 

[ceylonfj40nut]

Have you gapped your booster push rod to master cylinder? Did you use the tool to do it?