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11/23/2017 4:32 PM  #1


69 disc/drum question

  I have a 69 Cougar with MS 12.5" front disc conversion and 2" rear drums. I have a 2000 V6 master cyl. with the flat style tank. It's plumbed into a Wilwood  adj proportional valve and 10# residual valve.  I'm now able to drive the car, and the pedal is the original non power drum/drum. It is bled well, with about 3/4" of travel before hard pedal. I have the proportion adjuster backed off (counter-clock wise) to the max.Which, I believe is max front bias. It will stop, but needs a lot of foot pressure. Even, slamming the pedal will not lock up the tires. Do I need a smaller 15/16" or 7/8" master or is something else wrong?

 

11/23/2017 8:05 PM  #2


Re: 69 disc/drum question

It sounds like your brakes are working as I would expect. You have a good combination with those discs and drums. That is a good master cylinder. The prop valve should be good, if it works at all, lots of folks seem to have trouble with that brand. You have a manual brake pedal with manual brakes. Drum brakes work well with 300-500 psig hydraulic pressure which you should be able to generate with your system. Those disc brakes are going to need 1200-1500 psig hydraulic pressure which is going to be hard to generate with your system. The usual solution to generate more pressure is a booster, but if you want to stay manual, then yes you must reduce the diameter of the master cylinder. The downside of a smaller master cylinder is that more pedal travel is required for the same volume of brake fluid. You will have to make sure the brake pedal does not bottom out.

Out of couriousity, why were you trying to lock your brakes? Locked brakes reduce steering control, plus a hot brake pad clamped solid against the rotor can transfer pad material to the rotor making a “high spot” (admittedly almost unmeasurable) that could cause vibration during braking.

 

11/23/2017 8:24 PM  #3


Re: 69 disc/drum question

Id add a power brake booster and power brake pedal.  It will make it stop a lot nicer.

 

11/23/2017 9:37 PM  #4


Re: 69 disc/drum question

Also see the post on the HowTo page about bedding brake pads before doing any hard stops. The tendency is typical when changing from drums to discs to go try and lock them up, which typically ruins the pads instantly. Also, brake pads with LIFETIME WARRANTY are usually so hard they do not stop well on older cars that either have no power brakes or power brakes that are not as powerful as modern boosted brakes.

If neither of these suggestions work, then reduce to 15/16” mc OR install correct booster and pedal for the car. That would be a 69 Bendix booster and pedal with 3:1 ratio like factory pdb cars used.  Available thru mustangsteve part number pb-6770-mt for manual trans or pb-6770-at for automatic transmission cars.  It in cludes pedal and booster.


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