[DartGTS]

I just replaced the master cylinder on my '68 Dart GTS. I also replaced all the fluid with first Castrol LMA fluid then replaced it with Valvoline Synpower fluid. Both are DOT 3 & 4 type fluids. Now when I drive the front brakes (disc) will gradually apply once the car is warmed up to the point of actually stopping the car. If I want to get going again I have to wait for the car to cool for a while or crack open the bleeder screws and relieve pressure in the calipers. The brake pedal also gets real stiff at the same rate that the brakes apply. When I open the master cylinder top while the brakes are hot the fluid level remains the same as when I filled it. What is happening and why didn't it happen with the old master cylinder? How can I fix it? I've tried heat shielding on the master cylinder and other areas of the brake system, and replacing all the fluid with a different brand with no success. BTW the car has a 440/auto (not factory) with headers.

[Tim240Z]

Sounds like the fluid is not returning to the master after brakes are applied.....check for kniks in the lines, or other valving problems...

[adtkart]

Have you checked to see if it is all wheels, and not just one? Could it be a caliper is sticking?

[Frisco]

My son had this problem when he changed the master cylinder in a Jeep he had. Turns out that the master cylinder he was sold as a replacement was incorrect and needed a shorter pushrod from the brake pedal. The original pushrod was about 3/8" longer than required for the master cylinder, causing the front calipers to drag. This in turn heated the fluid to a point where the front brakes would lock up after driving just a few (2-3) miles. Roadside repair was to space the master cylinder away from the firewall (used some nuts and longer bolts) so he could get back to my house. I shortened the pushrod and re-mounted the master cylinder correctly.

Easy check for you. With everything cold, check for freeplay between the brake pedal pushrod and the master cylinder. If there is none then that's your problem.

[DartGTS]

Thanks to all those who responded. The problem seemed to lie in the master cylinder valving. Damn cheap rebuilds. It's hard to find a qualiy rebulit product anymore.