1969 Power Disc Brakes--What controls pedal sponginess?

[DeadStang]

Of #3 1969 Mustangs, all with power disc brakes:

(1) One has a hard pedal, but the car stops fine (maybe not great in a panic situation)...I believe this is a symptom on a bad brake booster. Is this correct?

(2) My sig pic Mach has GREAT brakes. I personally replaced the master cylinder when the brake pedal had to be "pumped up" to get it to stop, but the rest of the system was fairly new when the car was killed a few years back. It has two new rear wheel cylinders as well.

(3) My newest is a GT that just came back from the shop today. The brakes work fine, but the pedal depresses much closer to the floor than in the Mach, as in it feels spongy-ish to me, but does not require pumping. It has two new wheel cylinders and much of the system was replaced my them (at the shop) and the brakes bled, but the booster and master cylinder were not replaced.

My questions--what controls the "feel" of the pedal, if a hard pedal is the booster being bad, a soft pedal with no leaks in the system must be a weak master cylinder? If I want the brakes in my GT to be similar to the Mach, should I replace the MC? Or do different cars after 40 years, assuming no defects in the system, just perform differently?

 

[Starfury]

Sponginess means there's air in the system. And I agree, the first car sounds like a booster that's not boosting, either because it's bad or because it's not getting proper vacuum.

 

[daveSanborn]

Check to make sure that the rear drum brakes are self-adjusting properly. If the self-adjusters are not working properly, i.e., setting the proper amount of clearance between the static shoe and rotating drum, the pedal will go much further to the floor than normal. How the self-adjusters are set will make a big difference on the "feel" of the brakes.

I agree that "spongy" brakes is usally air trapped in the system. I've gone through well over a gallon before after installing all new lines components.

 

[Sluggo]

Of #3 1969 Mustangs, all with power disc brakes:

(1) One has a hard pedal, but the car stops fine (maybe not great in a panic situation)...I believe this is a symptom on a bad brake booster. Is this correct?

(2) My sig pic Mach has GREAT brakes. I personally replaced the master cylinder when the brake pedal had to be "pumped up" to get it to stop, but the rest of the system was fairly new when the car was killed a few years back. It has two new rear wheel cylinders as well.

(3) My newest is a GT that just came back from the shop today. The brakes work fine, but the pedal depresses much closer to the floor than in the Mach, as in it feels spongy-ish to me, but does not require pumping. It has two new wheel cylinders and much of the system was replaced my them (at the shop) and the brakes bled, but the booster and master cylinder were not replaced.

My questions--what controls the "feel" of the pedal, if a hard pedal is the booster being bad, a soft pedal with no leaks in the system must be a weak master cylinder? If I want the brakes in my GT to be similar to the Mach, should I replace the MC? Or do different cars after 40 years, assuming no defects in the system, just perform differently?

(1) Correct. Vacuum leaks cause a similar effect as stated by Starfury.

(2) Sounds like bleeding may solve this one. Also look for brake hose/line problems. I had a similar problem with mine. I could not get the rear to bleed properly. Turned out to be the rear hose was collapsed inside and plugged. New hose and bleeding fixed it.

(3) I agree with Dave. Pedal height will be affected by the adjustment of the rear shoes. You may need to bleed and check that hose also.