brake pedal too firm with 87+ front brakes/rear disks

[dk5_cobra]

last week I finished my first stage of brake upgrades for my 79 cobra and it left the pedal much too firm to stop the car quickly even at low speeds. (hasn't been on the road, just in the driveway)

my setup consists of:

87 gt brake booster (adjusted to mc)
1985 lincolin tc 1" bore mc
thunderbird turbo coupe rear disks (27mm caliper piston I believe?)
87 gt 11" front disks with standard 60mm calipers/87 spindles/stainless lines/solid caliper bushings
adjustable prop. valve/gutted stock valve

the car does have a fairly agressive cam that will pull about 11-12inhg of vacuum at idle so that could be part of it. and I'm thinking that the master cylinder is slightly oversized for this setup but I'm no expert in the matter.

The reason I decided to stay 4 lug for now was my current wheel/tire setup that I have and I just wanted a good place to start with the brakes to get the car drivable (has sat as a stripped shell for 20 years)

if anyone has suggestions for me, please chime in

thanks, Devin

[lilpanther]

last week I finished my first stage of brake upgrades for my 79 cobra and it left the pedal much too firm to stop the car quickly even at low speeds. (hasn't been on the road, just in the driveway)

my setup consists of:

87 gt brake booster (adjusted to mc)
1985 lincolin tc 1" bore mc
thunderbird turbo coupe rear disks (27mm caliper piston I believe?)
87 gt 11" front disks with standard 60mm calipers/87 spindles/stainless lines/solid caliper bushings
adjustable prop. valve/gutted stock valve

the car does have a fairly agressive cam that will pull about 11-12inhg of vacuum at idle so that could be part of it. and I'm thinking that the master cylinder is slightly oversized for this setup but I'm no expert in the matter.

The reason I decided to stay 4 lug for now was my current wheel/tire setup that I have and I just wanted a good place to start with the brakes to get the car drivable (has sat as a stripped shell for 20 years)

if anyone has suggestions for me, please chime in

thanks, Devin

Pretty sure you need a bigger brake booster. Usually when people change to front and rear discs they use either a SN-95 booster or a 1993 Cobra booster.

[DrewM]

Most people I know have stuck with the stock booster, the 93 Cobra booster offers a little more linearity and really no extra 'boost', IIRC the SN95 is similar in that regard. Basically it's not worth the hassle of installing the 93 Cobra booster for most people. Now that doesn't mean the factory booster isn't failing or has failed (or as you said the cam/engine setup isn't letting the booster do the work), that's possible. A 1" MC should be fine for the setup, it's basically what the 93 Cobra had (1" MC, 60mm front calipers, turbo coupe rears).

[wraithracing]

I would double check all your work before considering a new booster or any other parts.

First do you have plenty of vacuum to operate the brake booster properly. Generally anything under 16 inches of vacuum causes poor performance. Verify with an actual vacuum gauge and address as needed. Is the brake booster in good condition and working properly if you have full vacuum.

Are you positive that you adjusted the booster pushrod correctly. If the rod is too long or too short it will affect the pedal feel and the brake performance.

The last question, is there a difference in the pedal feel with the engine running compared to the engine off. If there is, then you are getting some power assist from the booster if there isn't then the booster is most likely your issue. If the power assist appears to be there and you still have little pedal feel, then most likely you have an issue with the M/C pushrod adjustment, the Master Cylinder, and or your brake bleeding.

Good Luck!

Trey

[dk5_cobra]

everything has already been replaced. brand new parts all around. I guess I just need to check my adjustment on the pushrod. is there a verifiable method for adjustment?

[qtrracer]

I guess I just need to check my adjustment on the pushrod. is there a verifiable method for adjustment?

Go to MM's site and read one of the installation instructions sets for M/Cs. There will be a procedure therein for the rod adjustment.