Brakes Hard Brake Pedal

Swhitney

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Feb 11, 2019
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I did the 4 to 5 lug conversion 10 + years ago and have always had a very firm pedal, and it really sucks. Is there something I can swap to make the pedal less firm and so that brakes are more responsive?

1992 Mustang GT
347 stroker

>13 inch front rotors
>cobra calipers
>11.65 rear disc
>cobra calipers
>all SS brake lines
>Brake booster I think is out of a 94+ car
>stock MC
>insert for proportioning valve in to the MC
 
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90sickfox

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You need an adjustable proportioning valve, like wildwood. For 4 wheel discs you should have either a 98 ( 15/16 bore ) master cylinder or a cobra ( 1 inch ) master cylinder. Need to verify which booster you have. A pic would be great. If unknown you need a 93 cobra booster.

Check your vacuum at idle. My vacuum is only 8in.hg. at idle. ( comp xe282hr ) I had to add an electric vacuum pump and relay from a Chevy cruze to keep vacuum at the booster you could also use a vacuum reservoir connected in the hose for the booster. Before this I could not lock the brakes without a huge amount if force after first brake....the third time they wouldn't lock at all. I like the electric booster because I have full brakes with the engine off, key on.
 

Swhitney

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Feb 11, 2019
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I am running the mustang brake proportioning valve plug from LMR.

Here's a pic of the booster and MC. Doesn't seem like much help, as there is no markings on it except 13088.

Photo Aug 17, 10 19 03 AM.jpg

 

2000xp8

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When you say stock MC in your first post, do you mean stock foxbody master cylinder?
 

90sickfox

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That looks like a stock v8 brake booster.

The stock master cylinder has 3 lines coming out.....I think. That may be a cobra master cylinder.
 

Mustang5L5

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That's not a stock master cylinder. It is a stock booster though.

Without knowing which MC that is, it's tough to say if it's correct. I ran a 1" 1993 Cobra MC with an 93 cobra booster with that same setup and the braking was perfect.
 

Swhitney

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Feb 11, 2019
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That's not a stock master cylinder. It is a stock booster though.

Without knowing which MC that is, it's tough to say if it's correct. I ran a 1" 1993 Cobra MC with an 93 cobra booster with that same setup and the braking was perfect.


Well I can say that my pedal is firm as hell. At this point I think I need to also upgrade to the 93 cobra master cylinder and also the above components. Might as well do it right the 2nd try. :)
 

Mstng93SSP

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It's your booster. That is a stock foxbody booster. You need a 93 cobra booster. Swap that booster and let me know how the pedal feels. I have done over 10 cobra brake conversions. The one time a stock booster was not swapped out for a 93 cobra booster the pedal was hard as hell and stopping during panic situations was extremely stressful. Change your booster.
 
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nickyb

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Agreed,change that booster, I used 95 booster and master and my brakes have been fan fucing tastic. I daily drive mine and I am very confident in my brakes.
oh I did not gut and plug my proportion valve, I gutted and used innards from a 95 valve.
good luck with yours.
 
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90sickfox

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I used to use the prop valve from a 98 and theater with stock booster. It made emergency braking sphincter puckering to say the least. The 93 cobra booster was by far one of the hardest but most rewarding things I've swapped in this car.
 
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Swhitney

Active Member
Feb 11, 2019
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Oregon City
Agreed,change that booster, I used 95 booster and master and my brakes have been fan fucing tastic. I daily drive mine and I am very confident in my brakes.
oh I did not gut and plug my proportion valve, I gutted and used innards from a 95 valve.
good luck with yours.

I remember changing my booster to a newer model mustang, but cannot remember which one it was from. I'm going to try swapping a few key items out mentioned above, then go from there. I know those components need to be change anyways since they are currently not correct. Parts have been ordered from LMR, and I still think my master cylinder is not a 93 mustang cobra model, which is required per LMRs install instructions.
 

manicmechanic007

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If you drove it into my shop... Technician A says test drive and verify the concern. Test engine vacuum at booster. Perform booster test. The possible mismatch of parts and the size of the pistons matters but your hard pedal is more likely caused by booster failure or inadequate vacuum. You want 16 inches of vacuum at least at idle. The booster test is as follows. Press hard on brake then start the engine. The pedal should drop as the engine vacuum gets to the booster. In your case you may want to do this test with a vacuum gauge installed. The gauge will let you see what is happening engine vacuum wise. If your cam has a ton of overlap you will not build much vacuum and you will need a pump
 

manicmechanic007

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IMHO 90sickfox is on to it due to the pedal being hard for ten years. I would like to see you get the correct MC and the adjustable proportioning valves are great. How long you had the stroker? 10 years?
 

Mustang5L5

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Well I can say that my pedal is firm as hell. At this point I think I need to also upgrade to the 93 cobra master cylinder and also the above components. Might as well do it right the 2nd try. :)

Change the booster first.

Your master might be the right one (93 cobra) for you so i wouldn't blindly change it yet.
 

Mustang5L5

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I remember changing my booster to a newer model mustang, but cannot remember which one it was from.

That's a foxbody booster for sure.


Boosters you can use:

1993 Cobra: Best one to use since the stud pattern is fox-body correct and the studs are SAE

1994-1995 Mustang: Same as cobra except metric studs. One stud is offset, so you will need to slot for that stud. Simply make a template.

99-04 V6 Mustang (same as 94-95), although due to some uncertainties on various dimensions i can't recommend this one fully yet.


No not use a 96-98 V6 booster. The brake pedal rod angle is different and not compatible. 93 Cobra boosters are hard to find right now, so i'd grab a 1994 booster
 

90sickfox

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IMHO 90sickfox is on to it due to the pedal being hard for ten years. I would like to see you get the correct MC and the adjustable proportioning valves are great. How long you had the stroker? 10 years?

This stroker has been about 6 w/ IRS and Cobra calipers. I've been 5 lug with an 04 IRS and 98 front calipers since 2008 or 9. Was 5 lug w/ 98 calipers and disc rear for about 5 years before that. Lincoln rotors and 5 lug axles /drums for about 5 years before that. Four different engines with the IRS- 302, 306, 347, and 331. It was drivable but wouldn't apply enough assistance.

Oh...and 2 different chassis.

I've been known to drive death traps in my earlier years. It was all part of the cheating death part. Drive fast everywhere and you won't feel it when it happens. SMH

It could be as simple as a bad vacuum check valve. The bigger booster will help in any combination. The master cylinder depends on the area of the calipers and how the driver wants the pedal to feel.

Technician B says; The correct booster test also involves it holding pressure to give assist after the car is turned off. Full assist for one braking event and partial for the second within a set time frame.