Classic Mustang Disc Brake Conversion issues

[Max Power]

First off, i know this is not necessarily a classic mustang site, but i will ask anyway as there seems to be some smart people on here.

So, about a year ago, the 65 mustang had a SSBC disc brake conversion done. The main reason why is because the existing drum brakes were not working well at all (fronts pulled to one side and one rear was dragging). They had bad stopping power in an emergency stop, but were OK otherwise. SO since the conversion was done, the new brakes just haven't worked. Here is the setup.

SSBC 4 piston 13" up front (single piston from SSBC listed as for 6 cyl mustangs)
SSBC rear discs, single piston
Brake booster
Master cylinder with 1" bore
Bias valve
brake pedal is a cantilevered setup (says the mechanic)
Also added a electric vacuum pump because engine vacuum was a bit low at 10".
engine is a 351 Cleveland with 4 BBL carb (i think it is a 71 non mustang motor)
4 speed manual transmission
(car was originally a 289 auto) previous owner did the swap.
9-inch rear, not sure about gears
The problem is that at the front calipers, there is only 350 to 400 psi of pressure, which SSBC states 1200 psi is needed.

The shop it is at now, who is not the install shop, says that everything is correctly connected and the system has been bled numerous times.

SSBC hasn't really been any help as all of their suggestions have already been accounted for (correct hook up, bench bled master cylinder etc.)

Now i think that even with manual brakes, without Booster, we should be able to generate more than 350PSI at the calipers.

Does anyone have any suggestions or thoughts? Any help would be greatly appreciated.

Thanks - Chris

[PaxtonShelby]

I agree that there are some smart folks here on SCS. That being said, I think you'd find some help if you posted this on www.vintage-mustang.com .

These might be dumb questions, but I figure it is best to start with the basics.

Is the master cylinder the exact one recommended by SSBC?

I assume you're running Dot 4 fluid, and not the crap Dot 5 ( silicone ) fluid?

Not sure what you mean by a cantilevered setup? Was the factory pedal assembly modified in any way or replaced with a pedal assembly from another model?

By "bias valve" do you mean an adjustable proportioning valve? Is it installed in the rear hard line?

Do you have an adjustable master cylinder pushrod?

My thought is that, even with the 1" bore MC, maybe you're not moving enough fluid - either because of the brake pedal configuration, the brake pushrod, insufficient vacuum, or a combination of those.

Let us know what you find out. Good luck!

[somethingclever]

Pressure = force/area


Therefore smaller bore m/c equals more line pressure.

Area of 7/8" bore is .601 in^2.

Area of 1" bore is .785 in^2.

The difference of these two is .765.

350PSI/.765 = 457PSI if going to 7/8" bore

Still a long way from 1200.



Tell me about your brake hoses.

[Max Power]

Hey guys, thanks for the replies. It looks like the biggest problem may be the brake pedal. it does not appear to be a 6 to 1 ratio setup. I am going to measure in a day or two to find out. I drove the car home from the shop, which straightened out some lines and removed the proportioning valve and added the vacuum pump. the pump is now working properly so there is enough vacuum on the booster. i was actually able to get the brakes to lock up from 30 mph, which i couldn't do before, but it took a lot of effort. so between the pedal adjustment (if possible) and maybe a dual diaphragm booster, i think we might be OK.

Once again, thanks for the suggestions, they led me to this article about brake setups, it is a very good read.
http://www.hotrodheaven.com/tech/brakes/

[map351]

The old conversion trick on the early cars is 4 piston Kelsey-Hayes caliper or Granada disc & a 1970 Maverick Manual Disc master local parts store you don't need a vac booster, good pedal feel & lock en at will. I used that combo on my cars & Ranchero for years.