Installed Cobra brakes, still can't stop. Going hydroboost

1slow95 said:

I know the stock MC is why the pedal is hard, but wouldn't the larger bore MC equal more pressure? Basically, the stock MC has the same braking power at half pedal as the Cobra MC at 3/4 pedal?

Click to expand...

No, that's not how it works. Bigger is not always better as in this case.

Basically, part of the cobra brake conversion on a 94-95 is that you NEED a new master cylinder. I'm surprised at just how many 94-95 guys don't realize this. Most guys complaining that the Cobra brakes weren't that great on their GT's after the swap never changed the MC out.

Even Ford engineers recognize this which is why the M-2300-Q kit was sold. It's Cobra 13" brakes with the new required master cylinder

Ford Racing M-2300-Q Ford Racing Mustang Brake Upgrade Kits


Reason you need a smaller bore MC here is because, beleive it or not, the cobra calipers are SMALLER than the stock 66 units you replaced.

Here some numbers

2x 66mm calipers = 6842mm^2 of piston area

2x 99-04 Cobra/Mach/Bullitt 40.5mm calipers = 5152mm^2 of piston area.

2x 94-98 Cobra calipers 38mm = 4536mm^2

In terms of hydraulics, that's a HUGE differnce. You are decreasing the slave cylinder (caliper) piston area and making the master and slave closer in ratio. That requires MORE force from your leg to achieve the same braking pressure. In order to compensate mechanically, you need to reduce the size of the MC's bore in order to keep the same appropriate hydraulic ratio.


This is where things get tricky. The 94-95 Cobra 15/16" MC was developed for the 94-98 Cobra calipers which feature 38mm pistons and an even smaller surface area of 4536mm^2. The 99+ casting of the 13" Cobra/Bullitt/Mach caliper has more surface area, so use of the 15/16" MC may result in a soft pedal with a long travel.

The 94-95 GT MC is 1 1/16".

I basically have the same setup as you do on my Fox Mustang. Mach 1 front calipers, Mach 1 rears. I ran the 94-95 GT MC (because it's what i had at the time) and the brakes SUCKED. I swapped to a smaller 1993 Cobra MC 1" bore (no brake lines need to be changed...direct plug in) and the braking was MUCH better. The 15/16" MC is ideal for the 94-98 Calipers, but i went with the 1" bore because the 99-04 versions have more piston surface area. I like the pedal feel with the 1", some guys on the fox boards like that setup with the 15/16" MC instead.

To swap in a 94-95 Cobra 15/16" MC, you will need a $20 brake line adapter kit from maximum motorsports. MMBAK-7

Master Cylinder Adapter, 94-95 Cobra master cylinder, 94-95 with Stock Proportining Valve [MMBAK-7] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!

The 1993 Cobra 1" MC is a direct swap in with no line changes needed.

I'm not telling you which MC you should go with, but just stating that the MC NEEDS to be changed when installing 13" brakes on a 94-95 GT. Without it, you won't be able to put as much hydraulic force into the calipers and then what's the point of the upgrade?

Hydroboost does offer way more assist than vacuum brakes, but you'll need to swap the booster, MC, run new lines and run the power steering hoses, etc. The gt master cylinder is 1 1/16, but the cobra and mach use the same size in hydroboost. A master cylinder swap is MUCH easier. Yoir vacuum numbers are idle are not that critical as overall vacuum at cruise. If you can pull good vacuum during cruise, it should build up even stored vacuum for several stops and your brakes should work fine.