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66 Drum brake issue

3K views 19 replies 5 participants last post by  1carguy 
#1 ·
Folks we have run into a brake issue on a 66 rebuild. Everything related to the brakes has been replaced. All drum brakes with the only upgrade being a dual bowl master cylinder. We have tried to bleed the brakes three times with the same end result of a soft pedal. What takes place during the bleeding process is that when bleeding the front brakes everything in the process seems normal in that the pedal is pumped up, held and when the fluid is bleed the pedal goes to the floor- all as it should be. When bleeding the rear brakes the pedal never goes to the floor yet fluid is being expelled. We can not figure out why the rear brake process is differing from the front and does this have something to do with the soft pedal. Any input greatly appreciated.
Bob
 
#2 ·
The rear brakes don't take much fluid and your MC is probably set up so that complete failure at the rear will not disable your front brakes; i.e. you still have some pedal left no matter what happens to the rear. That wouldn't necessarily have anything to do with a soft pedal.
 
#3 ·
Thanks Ivy, do I understand correctly by stating that rear brakes do not fade to the floor under pressure while bleeding because the front brakes are holding and preventing total lost of pedal? It that is the case why do the front brakes fade to the floor under pressure, wouldn't the rear brakes prevent that fade to floor? I must still be lost.
Bob
 
#4 ·
Thinking about it, you may have an inappropriate MC.

You are pumping two MCs with the same pedal; one in front of the other. During the bleeding process you are simulating failure in each of the systems to run fluid through the lines. If the total piston travel is different for the two pistons then one can allow the pedal to reach the floor and the other one will not.

If either one has travel that large then you have defeated the purpose of the dual MC. If your front circuit fails and reaches the floor you won't be able to generate any pressure in the rear since your pedal is already on the floor. Hopefully your rear circuit isn't well bled and you reach the floor from a combined movement of both pistons.

Is this MC one designed for your car? Not a good question since a '66 never had a dual MC. Is this MC one designed to work with your car's braking system?
 
#5 ·
Ivy I trust its correct since I ordered it as an upgrade kit from CJPony. It came with the necessary tubes for the conversion. The MC is listed as BMC32 Cardone Select New Brake Master Cylinder for all 1967, 1968, 1969 and 1970 Mustangs with Manual Drum Brakes. I followed their installation video tutorial but could have done something wrong. Thanks, Bob
 
#11 · (Edited)
In doing the conversion the length of the MC rod is critical, and must be right, in addition the forward section of the MC goes to the rear brake line, the rearward section of the MC goes to the front brakes. If everything has been replaced it takes time to seat the shoes to the drums, also the manual brakes should have been adjusted manually so that they just barely drag,when spinning a wheel. The reason the pedal will not go to the floor is the reason you went to a dual MC, so that when there is a loss of pressure on one axle the other still has some braking action. You should have bleed the rears first, always start bleeding at the farthest wheel from the MC and work to the closest. Once bleeding is done, redo it in a couple of days, as many times there is a small amount of air in the lines, that will work to the top and can then be bled out. And a MC for a 67 is right, I converted mine over 40 years ago, but I also used the pedal assembly from my 67, since I was converting from a automatic setup to the manual transmission setup. My 2 cts from an old mechanic. Good Luck.
 
#13 ·
...I converted mine over 40 years ago, but I also used the pedal assembly from my 67...
That might allow you to use a different MC than if you were using a '66 pedal assembly.

I don't know the part numbers but sixfifty may be right about the differing numbers. Whatever it is, when you are done your pedal can't go to the floor when bleeding either circuit by itself. For a single bowl MC it doesn't matter if the pedal goes to the floor; your brakes have failed either way.
 
#12 ·
Thanks "old mechanic" it is as you mentioned, forward MC chamber to rear and rear MC chamber to front. As mentioned earlier we used the same shaft that was in the original MC. Does an "old mechanic" have any idea why the safety feature of the dual bowl work when the rear brakes are being bleed but when the fronts are being bleed they go to the floor.
 
#14 ·
The front brakes do most of the braking, and you rarely want the rears to be able to lock up, unless all four will lock up at near the same point, otherwise the rear of the car comes around. Also sometimes, I have had to leave a bleeder open on the fronts while I bled the rears on some cars, because I could only get a tiny amount of brake fluid from the rears and couldn't get them bled, then once the rears were done, then I re-bled the fronts. Don't forget about what I mentioned about adjusting the brake shoes manually, until they slightly drag, because if some are adjusted too loose, it will require more fluid from the MC to get them to work. Good luck.
 
#17 ·
Dave, I got stopped in the middle by a call from the upholsterer and had to get the car over there for the top and seats. Here is what I have learned: Bench bleeding is paramount to start and I think that was part of my problem in that we did bleed the MC but maybe not enough. My pedal is firmer and I was about to bleed the wheel again when I got the call. I don't have a vacuum pump so it requires two guys and I was one guy short. The only remaining concern I have and could not get an answer on is the following:
Some say while bleeding the pedal should go to the floor during the process, others say no, it won't go to the floor because you are using a dual well MC and when you fluid out on the rear the front is still holding. Makes since but-
When bleeding my rear wheels the pedal will hold and not go to the floor. On the other hand when bleeding the front wheels the pedal does go to the floor. That is were I currently stand. After bleeding the MC and getting some air out I didn't get the chance to go back and bleed the wheels to see if there was a change. I am going to the Ford Nationals in Carlisle this week so will be searching for an answer. Sorry for the gabby reply
Bob
 
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