[broncos16]

I have a 1971 mustang coupe with front disc/rear drum brakes. The master cylinder was recently replaced to hopefully remedy a small leak in the old one. Now, the brakes are worse than ever. The pedal is very spongy, and even when pushed to the floor the car only coasts to a stop (if i'm lucky). It's nearly un-drivable in it's current state. There is also a leak between the cylinder and the firewall. But this can't be the only problem because it has always done that, even with the old cylinder in place.

 

[jaguars_fan]

That spongy feel usually means air in the lines. Another consideration is the master cylinder itself. I cannot recall on the Mustang but the last master cylinder I bought I had to specify disc or drum up front as they are different.

As for a leak, the only thing running through the firewall is the push rod so the leak should be something other than brake fluid.

Mac

 

[Lizer]

After the new master cylinder was installed, were the brake lines bled? If they were not bled then you would expect to have this problem because there is air in the lines. Bleed the brakes and your brakes will be back to normal.

Regarding the leak...it still can be brake fluid and I've had master cylinders leak in that area before against the fire wall. If a rear seal on the master cylinder is bad, brake fluid can leak out around it through the push rod bore and run down the firewall.