help, low brake pedal only while running

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help, low brake pedal only while running

April 29, 2017, 09:15:29 PM

hey guys,

so I swapped in a tc rear in my bird (I already have 11 front also) used a 94 cobra(15/16) master and drove it. it drove ok, but had a kinda low pedal. So I wanted a firmer pedal and got a 94 gt master (1-1/16). I bench bled the master, installed, gravity bled the system, then got someone to help me force bleed it. my problem now is the pedal is firm with the engine off, but when i start it, the pedal goes to the floor, i have some brakes, but BARELY. its really not safe to drive. I've since rebled the master once more and the whole system about 4 times and it never gets any better. I'm considering putting the cobra master back on, but that doesnt make sense, how does a smaller master work better. the cobra was a kind of low pedal but it was definitely drive-able, the gt master is not. should I just keep bleeding it a bunch more? its also kind of weird if it is trapped air, that this air didnt get trapped when installed the gt. really need some help guys

thanks
Keith

help, low brake pedal only while running

Reply #1 – April 30, 2017, 10:32:28 AM

can anyone verify that the gt cylinder is bigger than the cobra. seems odd that the cobra would be smaller. and if going to the gt was downgrading it would explain how the pedal got worse. 50resto lists the specs and i'd assume them to be right but i'm just really scratching my head at this.

https://lmr.com/item/LRS-2140F/1994-95-Mustang-Cobra-Brake-Master-Cylinder
https://lmr.com/item/LRS-2140H/1994-95-Mustang-Gt-V6-Brake-Master-Cylinder

help, low brake pedal only while running

Reply #2 – May 01, 2017, 11:26:23 AM

Master Cylinder Piston Bore Data:

SN95 Mustang
GT MC bore = 1-1/16"
Cobra MC bore = 15/16"

Fox Mustang
Cobra MC Bore = 1"
GT MC bore = 21mm or about 13/16"
LX 4 Cylinder MC bore = 21mm or about 13/16"

Fox Thunderbird
Turbo Coupe = 24mm or right at 1"
All others = 13/16"

Large bore MC will have less pressure than the small bore MC but a higher fluid volume to fill larger bore calipers.

Front Brake Caliper Piston Bore Data:

SN95 Mustang
1999 GT/V6 = Twin 44.5mm
94-98 GT/V6 = Single 66mm
94-98 Cobra = Twin 38mm

Fox Mustang
All = Single 60mm (calipers will not physically interchange)

Fox Thunderbird
All = Single 60mm (calipers will not physically interchange)

To see the difference in the calipers we can do a little math and solve for area of a circle and then compare those numbers. Remember area of a circle = πr^2 so here we go:

SN95 Mustang
99+ GT/V6 = Twin 44.5mm = 3110.6 mm^2
94-98 GT/V6 = Single 66mm = 3421.2 mm^2
94-98 Cobra = Twin 38mm = 2268.2 mm^2

Fox Mustang and Thunderbird/Cougar
Single 60mm = 2827.4 mm^2

The 94-98 V6/GT single 66mm calipers will take the most volume to move at 3421.2 mm^2 of piston area and the MC that comes with those cars is 1-1/16".
The 99+ twin 44.5mm calipers will take the 2nd most volume to move at 3110.6 mm^2 of piston area and the MC that comes with those cars is again 1-1/16".
The Fox single 60mm calipers will take the 3rd most volume to move at 2827.4 mm^2 of piston area and the MC that comes with those cars is roughly 13/16".
The 94-98 Cobra twin 38mm calipers will take the 4th most volume to move at 2268.2 mm^2 of piston area and the MC that comes with those cars is 15/16".

Remember that the Fox Mustangs have rear drum brakes so that is the reason for the smaller bore MC with the exception of the Cobra due to it using the 45mm Varga rear calipers off the TC's. The SN95 Mustangs all have 38mm Varga rear piston calipers and again the TC has 45mm Varga rear piston calipers. If you look at the 1993 Mustang Cobra and the TC's they have identical braking systems hydraulically - 1" MC, 60mm front calipers, and 45mm rear calipers. The difference between them is that the Cobra has a larger booster than other Fox cars and I think the Teves II system on the TC's is a whole other can of worms that Ford should have never released so its hard to compare the boost on the MC's between the two.

With all of that being said I think your issue lies in the fact that you really should install the 93 Cobra MC and booster. I know several folks have had good luck with the SN95 15/16" and 1-1/16" MC's on the stock booster but it is not ideal. The other thing you need to do is get a pressure bleeder like this one as it will 100% eliminate air in the brake lines:

https://www.motiveproducts.com/collections/domestic-bleeder-kits/products/ford-asian-import-three-pr0ng-bleeder

One last thing is you may very well have a bad Cobra MC or are bench bleeding it incorrectly.

help, low brake pedal only while running

Reply #3 – May 01, 2017, 03:19:41 PM

cool, thanks for all that info! After messing with it further I am pretty much positive the master is bad. its just a advanced auto rebuilt one I'm sure it happens. but after seeing your post when I return it I well go in the middle with the 93 cobra master since like you said, I have 93 cobra brakes now. thanks again!

Keith

help, low brake pedal only while running

Reply #4 – May 01, 2017, 03:54:55 PM

If you are going this far I can stress to you how much I would advise also installing the 93 Cobra booster.