Brakes

[atcpilot]

Hello Cobra owners.

I purchased an already complete A/C Cobra kit car in June of 2008. I am very pleased with it except for the brakes. It is not leaking anywhere and I have had the brakes bled TWICE. The brake lines are metal tubing not rubber. The vehicle was built with a 1989 Mustang GT frame/engine etc. The brake pedal is very soft and travels all the way to the floor. But on rare occasions works normally. Do I need to change the Master Cylinder out? Is there a way to determine conclusively that that is the problem?

I am somewhat mechanically inclined but am stumped currently.

Can anyone give me any suggestions?

Thank you.

Steven

[strictlypersonl]

A few possibilities that I can think of:

Bad master cylinder
Excessive rotor runout
Loose wheel bearings
Caliper sticking in its support frame (floating design only)
It still might be air in the system. You didn't describe when you didn't have the problem.

It would help if we knew more about the details of your system.

[Gunner]

On the info given, I'd vote for a bad master cylinder with seals that are coming and going. But as Bob says, complete details on the system and when it works and doesn't will help.

Is the brake system from the supplier, a donor or somewhere else? Is it new or used parts?

[COBRA36]

Running a 512 lift cam robbing my vacuum for operating my brake booster I installed a electric vacuum pump , have a little better braking but I only have half pedal, any more suggestion brake have been bleed twice thank you (also going to shop talk fo help)

[Larry Spillman]

If it isnt air or a bad master cyl and you have drum rear brakes you may need a residual valve to keep a small amount of pressure in rear lines this will help brakes work better. Disc brakes work on pressure drum brakes work on pressure and volume it takes more fluid to open a rear cyl than it does to operate a disc caliper.If you have discs front and rear you may be boiling your brake fluid it can be getting to hot which will cause pedal to fade. Good Luck L Spillman

[Woodz428]

Originally Posted by Larry Spillman

If it isnt air or a bad master cyl and you have drum rear brakes you may need a residual valve to keep a small amount of pressure in rear lines this will help brakes work better. Disc brakes work on pressure drum brakes work on pressure and volume it takes more fluid to open a rear cyl than it does to operate a disc caliper.If you have discs front and rear you may be boiling your brake fluid it can be getting to hot which will cause pedal to fade. Good Luck L Spillman

I'm not a brake expert by any means, but build quite a few cars. I am pretty sure that the Disc brakes typically use more volume than drums...look at the comparative size. Many early cars had a stepped master when they went to optional front discs, so the master for the disc side was larger than the drum one.Usually they were larger in the rear( front side) or you couldn't machine or assemble it. Also drums are self energizing( they wrap themselves into the drum). The valve is to keep the drum shoe springs from pushing all the fluid back into the master, that's the purpose of the residual valve. While You are correct in the affects caused by not having one, most times it only is neccesary when the master is lower than the floor. There is also a porportioning valve that keeps the rear brakes from locking when you apply the brakes because the discs require more pressure because they only clamp and unlike drum brakes do not self energize.
Since it sounds like a Mustang donor type car, I'd lean toward the master cylinder or some other bad hydraulic component.

[Larry Spillman]

IF you take it to an expert he will fix it since you dont want to listen to advice from someone that has been building cars for 55 years. You may still be boilng brake fluid where it comes close to exhaust. wrap the headers in engine bay with heat tape and see what happens. you have to have and open mide to solve problems.Good Luck Again