Rear disk conversion issue

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my5thmopar

my5thmopar

Life Long MOPAR Owner
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I have a weird issue with my 64 D100 project. I have scarebird on front and rightstuff on rear. I have adjusted the parking brake. I used a tool to adjust the booster pin and it is not touching. There is no residual pressure valve. I have adjusted the proportioning for full pressure at rear. The master is a ssbc corvette style with booster made for a 69 charger. I’ve had this issue twice now that the rear brakes are dragging. I stopped during a 1.5 hour trip yesterday and the rears were hot. I first noticed the smell. The rear use the gm type calipers with the external parking brake. I completely released the parking brake and turned the pressure almost off to the rear. When I got home the rears were hot again. I’m a little lost. Any suggestions? Could the master or booster fail in a way to be engaging the rears? Craig
 
my5thmopar said:
I have a weird issue with my 64 D100 project. I have scarebird on front and rightstuff on rear. I have adjusted the parking brake. I used a tool to adjust the booster pin and it is not touching. There is no residual pressure valve. I have adjusted the proportioning for full pressure at rear. The master is a ssbc corvette style with booster made for a 69 charger. I’ve had this issue twice now that the rear brakes are dragging. I stopped during a 1.5 hour trip yesterday and the rears were hot. I first noticed the smell. The rear use the gm type calipers with the external parking brake. I completely released the parking brake and turned the pressure almost off to the rear. When I got home the rears were hot again. I’m a little lost. Any suggestions? Could the master or booster fail in a way to be engaging the rears? Craig
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The rear brakes mount using the various spacers to locate the mounting bracket? Could it be the wrong spacer was used? My right stuff rear kit came with three different spacers to move the mounting bracket and put the caliper in the right spot. Just a thought.
 
Yes, check spacing first. But If it is okay check to see if you had a sticky caliper or a bad intermediate brake hose, too. The parts may be new or near new, but my professional commission mechanic friends are getting overwhelmed by the run of bad over the counter parts right now. Start at the problem and work your way forward. And if you didn’t build your own master cylinder, it may be a good idea to double check the master that it doesn’t have a residual pressure valve if you haven’t already.
 
The kit was specifically for an 8 3/4 so no extra spacers.
Sticky caliper would have to be both. New hoses and I took the mc apart to clean and check for valve.
I’ll recheck the items suggested. This is a weird one. That’s why I was asking if this Could the master or booster fail in a way to be engaging the rears?
 
I wouldn't suspect the booster, but the M.C. could be responsible. You could also eliminate the proportioning valve to check it, but I would try the intermediate hose loose at the floor pan side and see if you can compress fluid from the calipers back through it. Oh, what kind of brake fluid are you running?
 
How would the master cause this? I see your idea about back flow through the lines. I’ll include that in my troubleshooting. The hose from the tee from the tube to the steel line is new too. I did make sure all the new hoses were clear. Dot3.
 
IIRC, '64 was still a tapered axle rear end (has a hub and drum held on by an axle nut) Maybe wrong kit for that rear?
 
Update. I checked clearance of the pads tonight and nothing seems to be dragging. The spacers are on per instructions. I checked the parking brake again and it is adjusted correctly. I talked with a older retired mechanic and his first suggestion like @Garrett Ellison , the hose on the diff. I now have a new issue with low pedal. I had the rear tires spinning in gear and the brakes would barely stop the freewheeling. Now it seems I have poor rear brakes. I have ordered a new master and rear hose. Besides, I never liked the bulky corvette master. I'll get my power bleeder out next and check fluid flow. I've been avoiding clamps and pressure gages but, it looks like I have to go down that path. I will report back. Craig
 
My original suspicion with the master cylinder was improper assembly or a bad part from the manufacturer. I know of one master failure that caused the same symptoms. Did the brakes slowly bleed off from locked or did you have to relieve the pressure? About all the bad brake hoses I've seen show no sign of have unraveled internally, and won't show it until you try to force back pressure through the hose. You can usually feel the soft spot or spots when you take it off and start working it, though.
 
Sill low pedal and I haven't gotten back to rears dragging but, I'm convinced it is all related. While removing the SSBC gm universal MC, I realized I had the connections to front/front and rear/rear. It seems this gm mc comes with a lot of the generic pwr brake conversion kits. The SSBC instructions don't even say how to plumb the lines for mopar. It states front or rear as if it doesn't matter. No RPV in the gm mc. I bench bled the mc again and get a geyser in the front and fluid rolling in the rear. I don't get geyser to the rear with a quick pump. I'm wondering if the mc is bypassing and bad? Comments? I don't have a high press gauge to check but, I'm going to buy one for Kubota problem. I bought the 80s something aluminum 2 cap Raybestos MC39178. The Mopar mounting is smaller and of course has the deep hole. I have slotted the ears on the mc and I'll fabricate a spacer tomorrow. It's worth a try anyway. I know some of the Wilwood use gm 3.375 mountings and I couldn't find a generic gm aluminum one. Any suggestions? Craig
 
I replaced the MC, readjusted the booster pin and I think the rear brake drag is fixed. I road tested and they lock up and release. My temperature gun and hand says no dragging. I have isolated the low pedal to the front, if my troubleshooting method is correct... MC was bench bled. I then attached the rear line which was previously bled with the power bleeder. Plugged the front. The brakes were high and hard. I put the front line on and they go almost to the floor. No leaks and everything nearly new. I bled the fronts old school with a bottle and helper. Truck leveled per MC. Plenty of fluid through bleeders and some air where I repositioned the banjos. I'm lost now.... if the pedal ratio is off, I think it still would have hard brakes. The MC is 1 1/8 which should be enough to get a hard pedal. Should I clamp the front calipers off at the same time, check pedal? Just looking for some suggestions. Craig
 
You state that it’s a low pedal height. I’m interpreting it that’s when you depress the pedal but it’s at normal height when at rest. Is the brake booster on a bell crank bracket or is it mounted directly to the firewall? Rather than just saying that it needs more volume and recommending a cylinder with a bigger bore, I’m going to ask what vehicle the adapted calipers were made for. You’re closer, but you probably are going to have to match at least the MC more closely to the calipers. Also, what size and type of brake lines do you have on it? My suspicion is it’s probably going to take this type of MC or maybe larger.
Allstar Performance ALL41064 Allstar Performance Brake Master Cylinders | Summit Racing
 
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Correct. Low when applying brakes. It seems to stop ok…still testing. Has booster bracket and both pedal and pin are adjusted correctly. 95-97 Crown Victoria front rotors, '88-91 GMC 1500 front calipers. The rear rightstuff Cadillac. All new 3/16 steel. Should I clamp the front calipers off at the same time, check pedal? Just looking for some suggestions.
 
Those years of Crown Vic's only had one or 1.062 bore sizes. I would push bleed the system from the calipers back to make certain all the air was out of the system first. But what size bore did the SSBC universal master cylinder have? I think you primarily are down to a fluid volume issue at this point and that a larger bore size cylinder is in order. Most larger modern 4 wheel disc brake systems have at least a 1.25 to a 1.3 inch master cylinder bore.
 
Both mc 1.125. I really need some troubleshooting advice. I’m not buying more parts. Everything is new or practically new. I’ve fixed many brake and hydraulic stuff over the years but, I’m missing something here.
 
Okay, that sounds reasonable enough. I'm thinking your pedal is firm and high when the fronts are plugged because the fluid has nowhere to go (except around a possibly leaky piston seal). You may want to reverse the MC lines to see if it has any effect.
 
Update. My brakes are satisfactory and stop the truck but, I don’t like the pedal travel. I had one front caliper that was sticking and little to no grease. When I have either front or back connected, then pedal is extremely hard like it was with manual brakes. When I hook up both front and back, the pedal moves 2-3 inches. I know it moves some with PB but not that much. It sure seems like a fluid volume issue. I was looking at a 1 1/4 MC. I’ve also considered a 2 psi RPV for the rear disk even though I shouldn’t need one. Thanks Craig
 
I have rear discs in the dart. The fronts are 73+ A body single piston and the rear are 94 mustang Cobra csingle.pisyon calipers. The master is a 15/16 without a prop valve. All from Dr. diff.

Disc brakes do take more volume to work. Since you have power brakes, step up to the next size MC and see if you like that feel better.
 
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