A Body Disc Brake Question

[furz4444]

I recently rebuilt the front end on my 73 dart and replaced the calipers and pads due to calipers being stuck and would not retract for new pads. Rotors were not resurfaced as brakes had been done fairly recently. I could not get one of the calipers anywhere, no parts stores had them and they were backordered from the rebuilders with no in stock date. Can't remember which side right now. Anyway I ended up using 76 calipers on a 73 dart, they fit perfectly and were half the price. They do have the phelonic pistions which I was never wild about but they seem to work ok with only one issue. I have a much lower brake pedal then before, it's almost like I need more piston travel in the master cylinder or something. I did notice that the 76 used a different master cylinder and now I am wondering if I will need to change the master cylinder or something to be compatabile. Does anybody know the differences between the 73 and 76 brake systems that could shed some light on this. I was wondering if they might have changed the piston sizes in the calipers or master cylinders or both. I am going to re-bleed the front brakes and see if that helps any but I think I am missing something here.

[cageman]

rebleed it, that is probably the culpret, air.

[dave571]

Bleed should be your issue from the sound of it.

Did you do the rears at the same time? That would affect pedal height too.


The master cylinder used should have no affect on pedal height.

[George G. Leverette]

The calipers are different on the later A-bodies with front disc brakes, the later model calipers were the same on F. M, J ... bodies say a 76 Cardoba or 81 dippy or the Imperials so from 73-82 all of the calipersa are interchangeable. The thickness of the front brake pads differs and causes a difference in pedal height. If the car is stopping without abnormal action its probvably in spec and not a saafety issue. Bleeding all four corners when parts are changed is an excellent idea as previousally stated.

[dirttrackracer]

Not to be a pain in the %$#@ but could you explain how a differance in pad or rotor thickness affects pedal hight?

[dgc333]

Pad and rotor thickness do not effet the pedal height on disk brakes. If the rear drums are out of adjustment you may get more pedal travel.

Couple of things that may be causing the extra pedal travel.

1. Since the rotors weren't turned there may be some taper from the hub to the edge of the roto causing extra pedal travel to force the entire surface of the pad against the rotor.

2. The seal on the caliper piston distorts when the piston moves and tends to pull the piston back away from the rotor when the pressure is released. Thi si by design but you may be getting more of it happening, which should go away once the brakes are used some.

3. As already mentioned if there is now a mismatch between the bore of the caliper and the bore of the MC you can wind up with more travel. It would be the case where the MC bore is to small for the caliper bore.

[furz4444]

I did not bleed the rears and the front lines were off for several weeks, will be sure to re-bleed all 4 wheels. Am still wondering what the actuall differences in the hardware might be. My pedal does not feel spongy, and does not pump up like I have air in the lines, just seems lower. Will re-bleed using the two man method so I can see the fluid coming out, used the one man bleeder last time.

[John Kunkel]

If the pedal isn't spongy, air isn't the problem.

Most references show two different piston diameters used in '73-'76 calipers, 2.6" and 2.75" but the MC diameter is the same 1.32". If larger bore calipers were installed the pedal would be lower and, as mentioned earlier, if the caliper pistons retract farther than normal the pedal travel will also increase.

[furz4444]

Thanks for the info on the bore sizes. I wil go ahead and bleed the brakes and adjust the rears. If it still seems low I will pull the rotors and resurface.